Semiconductor memory device and memory system

ABSTRACT

A semiconductor memory device includes a first memory cell for storing data using at least three levels of threshold voltages, including a first level, a second level higher than the first level and a third level higher than the second level. A first word line is connected to the first memory cell. In writing of data to the first memory cell from a state where a threshold voltage of the first memory cell is the first level, a plurality of program operations and verify operations are performed, each program operation including applying a program voltage to the first word line, each verify operation including applying a read voltage lower than the program voltage. The program operations include a program operation for the second level and a program operation for the third level, and the verify operations include a verify operation for the second level, and do not include a verify operation for the third level.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Continuation Application of U.S. application Ser.No. 16/896,601, filed on Jun. 9, 2020, issued as U.S. Pat. No.10,916,300 on Feb. 9, 2021, which is a Continuation Application of U.S.application Ser. No. 16/733,491, filed on Jan. 3, 2020, issued as U.S.Pat. No. 10,714,170 on Jul. 14, 2020, which is a Divisional Applicationof U.S. application Ser. No. 15/702,476, filed Sep. 12, 2017, issued asU.S. Pat. No. 10,566,051 on Sep. 12, 2017, which is based upon andclaims the benefit of priority from Japanese Patent Application No.2017-029095, filed Feb. 20, 2017, the entire contents of all of whichare incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a semiconductor memorydevice.

BACKGROUND

A NAND type flash memory as a semiconductor memory device is known.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a memory systemaccording to a first embodiment;

FIG. 2 is a diagram showing an example of a circuit configuration of amemory cell array and a sense amplifier module included in asemiconductor memory device according to the first embodiment;

FIG. 3 is a diagram showing threshold distribution and data assignmentof memory cells included in the semiconductor memory device according tothe first embodiment;

FIG. 4 is a diagram showing an example of a circuit configuration of thesense amplifier module included in the semiconductor memory deviceaccording to the first embodiment;

FIG. 5 is a flowchart showing an example of a write operation of thememory system according to the first embodiment;

FIG. 6 is a diagram showing an example of a command sequence of thewrite operation of the memory system according to the first embodiment;

FIG. 7 is a diagram showing an example of a change in thresholddistribution in a first write operation of the memory system accordingto the first embodiment;

FIG. 8 is a diagram showing an example of a change in thresholddistribution in a second write operation of the memory system accordingto the first embodiment;

FIG. 9 is a diagram showing an example of a command sequence and awaveform of the write operation of the memory system according to thefirst embodiment;

FIG. 10 is a diagram showing threshold distribution and data assignmentof memory cells included in a semiconductor memory device according to asecond embodiment;

FIG. 11 is a flowchart showing an example of a write operation of amemory system according to the second embodiment;

FIG. 12 is a diagram showing an example of a command sequence of thewrite operation of the memory system according to the second embodiment;

FIG. 13 is a diagram showing an example of a change in thresholddistribution in a first write operation of the memory system accordingto the second embodiment;

FIG. 14 is a diagram showing an example of a change in thresholddistribution in a second write operation of the memory system accordingto the second embodiment;

FIG. 15 is a diagram showing threshold distribution and data assignmentof memory cells included in a semiconductor memory device according to athird embodiment;

FIG. 16 is a diagram showing an example of a change in thresholddistribution in a first write operation of a memory system according tothe third embodiment;

FIG. 17 is a diagram showing an example of a change in thresholddistribution in a second write operation of the memory system accordingto the third embodiment;

FIG. 18 is a diagram showing threshold distribution and data assignmentof memory cells included in a semiconductor memory device according to afourth embodiment;

FIG. 19 is a flowchart showing an example of a write operation of amemory system according to the fourth embodiment;

FIG. 20 is a diagram showing an example of a data conversion process inthe write operation of the memory system according to the fourthembodiment;

FIG. 21 is a diagram showing an example of a change in thresholddistribution in a first write operation of the memory system accordingto the fourth embodiment;

FIG. 22 is a diagram showing an example of a change in thresholddistribution in a second write operation of the memory system accordingto the fourth embodiment;

FIG. 23 is a diagram showing an example of a data conversion process ofa write operation of a memory system according to a variation of thefourth embodiment;

FIG. 24 is a diagram showing an example of a data conversion process ina write operation of a memory system according to a fifth embodiment;

FIG. 25 is a diagram showing an example of a change in thresholddistribution in a first write operation of the memory system accordingto the fifth embodiment;

FIG. 26 is a diagram showing an example of a change in thresholddistribution in a second write operation of the memory system accordingto the fifth embodiment;

FIG. 27 is a diagram showing an example of an internal data load using asoft bit in the second write operation of the memory system according tothe fifth embodiment;

FIG. 28 is a diagram showing an example of a command sequence and awaveform of the write operation of the memory system according to thefifth embodiment;

FIG. 29 is a diagram showing an example of a data conversion process ofa write operation of a memory system according to a variation of thefifth embodiment;

FIG. 30 is a flowchart showing an example of a write operation of amemory system according to a sixth embodiment;

FIG. 31 is a diagram showing an example of a data conversion process inthe write operation of the memory system according to the sixthembodiment;

FIG. 32 is a diagram showing an example of a command sequence of thewrite operation of the memory system according to the sixth embodiment;

FIG. 33 is a diagram showing an example of a change in thresholddistribution in a second write operation of the memory system accordingto the sixth embodiment;

FIG. 34 is a diagram showing an example of a circuit configuration of amemory cell array included in a semiconductor memory device according toa seventh embodiment;

FIG. 35 is a diagram showing an example of a cross-sectionalconfiguration of the memory cell array included in the semiconductormemory device according to the seventh embodiment;

FIG. 36 is a flowchart showing an example of a write operation of amemory system according to the seventh embodiment;

FIG. 37 is a diagram showing an example of a writing order in the writeoperation of the memory system according to the seventh embodiment;

FIG. 38 is a diagram showing an example of a writing order in the writeoperation of the memory system according to a comparative example of theseventh embodiment;

FIG. 39 is a diagram showing an example of a writing order in a writeoperation of a memory system according to a variation of the seventhembodiment;

FIG. 40 is a diagram showing threshold distribution and data assignmentof memory cells included in a semiconductor memory device according toan eighth embodiment;

FIG. 41 is a flowchart showing an example of a write operation of amemory system according to the eighth embodiment;

FIG. 42 is a waveform diagram showing an example of the write operationof the memory system according to the eighth embodiment;

FIG. 43 is a diagram showing an example of a change in thresholddistribution in the write operation of the memory system according tothe eighth embodiment;

FIG. 44 is a waveform diagram showing an example of the write operationof the memory system according to the eighth embodiment;

FIG. 45 is a flowchart showing an example of a write operation of amemory system according to a ninth embodiment;

FIG. 46 is a waveform diagram showing an example of the write operationof the memory system according to the ninth embodiment;

FIG. 47 is a diagram showing an example of a change in a thresholdvoltage in the write operation of the memory system according to theninth embodiment;

FIG. 48 is a waveform diagram showing an example of a write operation ofa memory system according to a tenth embodiment;

FIG. 49 is a diagram showing an example of a change in a thresholdvoltage in the write operation of the memory system according to thetenth embodiment;

FIG. 50 is a diagram showing an example of a change in a thresholdvoltage in the write operation of the memory system according to thetenth embodiment;

FIG. 51 is a diagram showing threshold distribution and data assignmentof memory cells included in a semiconductor memory device according toan eleventh embodiment;

FIG. 52 is a diagram showing an example of a change in thresholddistribution in a first write operation of a memory system according tothe eleventh embodiment;

FIG. 53 is a diagram showing an example of a change in thresholddistribution in the first write operation of the memory system accordingto the eleventh embodiment;

FIGS. 54A and 54B are flowcharts showing an example of a write operationof the memory system according to the eleventh embodiment;

FIGS. 55A and 55B are diagrams showing an example of a randomizationcircuit using M sequences in the memory system according to the eleventhembodiment;

FIGS. 56A, 56B, 57A, 57B, 58A, and 58B are flowcharts showing an exampleof the write operation of the memory system according to the eleventhembodiment;

FIGS. 59, 60, 61A, 61B, and 61C is a diagram showing an example of awriting order in a write operation of a memory system according to theeleventh embodiment;

FIG. 62, is a diagram showing an example of a change in thresholddistribution in the first write operation of the memory system accordingto a variation of the eleventh embodiment;

FIG. 63, is a diagram showing an example of a change in thresholddistribution in the second write operation of the memory systemaccording to the variation of the eleventh embodiment;

FIGS. 64 and 65 are flowcharts showing an example of a write operationof the memory system according to the variation of the eleventhembodiment;

FIG. 66 is a diagram showing an example of a circuit configuration of amemory cell array included in a semiconductor memory device according toa twelfth embodiment;

FIG. 67 is a diagram showing an example of a state and thresholddistribution of memory cells in the semiconductor memory deviceaccording to the twelfth embodiment;

FIGS. 68 and 69 are diagrams showing an example of a state and thresholddistribution of memory cells in the semiconductor memory deviceaccording to the twelfth embodiment;

FIG. 70 is an example of a table showing a reading method of a memorysystem according to the twelfth embodiment;

FIGS. 71 and 72 are waveform diagrams showing an example of a readoperation of the memory system according to the twelfth embodiment;

FIGS. 73 and 74 are examples of tables showing a reading method of thememory system according to the twelfth embodiment;

FIG. 75 is a waveform diagram showing an example of the read operationof the memory system according to the twelfth embodiment;

FIGS. 76 and 77 are waveform diagrams showing an example of a readoperation of the memory system according to the twelfth embodiment;

FIG. 78 is a diagram showing an example of a circuit configuration of amemory cell array included in a semiconductor memory device according toa thirteenth embodiment;

FIG. 79 is an example of a table showing a reading method of a memorysystem according to the thirteenth embodiment;

FIG. 80 is a diagram showing an example of a cross-sectionalconfiguration of the memory cell array included in the semiconductormemory device according to the thirteenth embodiment;

FIGS. 81A and 81B are diagrams showing an example of a planar layout ofthe memory cell array included in the semiconductor memory deviceaccording to the thirteenth embodiment;

FIG. 82 is a diagram showing an example of a cross-sectionalconfiguration of the memory cell array included in the semiconductormemory device according to the thirteenth embodiment;

FIGS. 83A, 83B, 83C, and 83D are diagrams showing an example of a planarlayout of the memory cell array included in the semiconductor memorydevice according to the thirteenth embodiment; and

FIG. 84 is a diagram showing an example of a cross-sectionalconfiguration of the memory cell array included in the semiconductormemory device according to the thirteenth embodiment.

DETAILED DESCRIPTION

In general, according to one embodiment, a semiconductor memory deviceincludes a first memory cell capable of storing 3-bit data. Whenreceiving first data including a first bit and a second bit from anexternal controller, the semiconductor memory device writes the receivedfirst data to the first memory cell. After receiving the first data,when the semiconductor memory device receives second data including athird bit and a fourth bit, the semiconductor memory device reads thefirst data from the first memory cell and writes the 3-bit data to thefirst memory cell based on 1-bit of the read first data and the receivedthird bit and fourth bit.

Hereinafter, embodiments will be described with reference to thedrawings. The drawings are schematic. In the following description, thesame reference signs denote constituent elements having substantiallythe same functions and configurations. Numeric characters after theletters constituting a reference sign, letters after the numericcharacters constituting a reference sign, and “under bar+letters”attached to the letters constituting a reference sign are referenced byreference signs containing the same letters, and are used to distinguishcomponents having a similar configuration. When the components denotedby the reference signs containing the same letters do not need to bedistinguished from each other, the components are referred to by thereference signs containing only the same letters or numeric characters.

[1] First Embodiment

A memory system according to a first embodiment will be described below.

[1-1] Configuration [1-1-1] Configuration of Memory System 1

First, with reference to FIG. 1, a configuration example of a memorysystem 1 will be described. FIG. 1 is a block diagram showing an exampleof the memory system according to the first embodiment. As shown in FIG.1, the memory system 1 includes a semiconductor memory device 10 and acontroller 20, and is connected to an external host apparatus 30.

The semiconductor memory device 10 is a NAND-type flash memory capableof nonvolatilely storing data. As shown in FIG. 1, the semiconductormemory device 10 includes a memory cell array 11, a command register 12,an address register 13, a sequencer 14, a driver circuit 15, a rowdecoder 16, and a sense amplifier module 17.

The memory cell array 11 includes blocks BLK0 to BLKn (n is an integerof 1 or more). The block BLK is a collection of a plurality ofnonvolatile memory cells associated with a bit line and a word line, andis, for example, a data erase unit. The semiconductor memory device 10can retain data by the Multi-level cell (MLC) method for storing data oftwo bits or more in each memory cell. In the present embodiment, it willbe described that the 3-level cell (TLC) method for storing 3-bit datain each memory cell is used.

The command register 12 retains a command CMD received from thecontroller 20. The address register 13 retains address information ADDreceived from the controller 20. The address information ADD includes apage address PA and a block address BA.

The sequencer 14 controls the overall operation of the semiconductormemory device 10 based on the command CMD retained in the commandregister 12. Specifically, the sequencer 14 controls the driver circuit15, the row decoder 16, the sense amplifier module 17, and the likebased on the command CMD, and executes a write operation, a readoperation, and the like.

The driver circuit 15 generates a desired voltage based on aninstruction from the sequencer 14. The driver circuit 15 furthersupplies the generated voltage to the row decoder 16 based on the pageaddress PA retained in the address register 13.

The row decoder 16 selects any one of the blocks BLK0 to BLKn based onthe block address BA retained in the address register 13. The rowdecoder 16 further selects a row direction in the selected block BLK,and applies the voltage supplied from the driver circuit 15 to theselected and unselected word lines.

The sense amplifier module 17 outputs to the controller 20 data DAT readfrom the memory cell array 11. The sense amplifier module 17 furthertransfers the write data DAT received from the controller 20 to thememory cell array 11.

The controller 20 instructs the semiconductor memory device 10 toexecute reading, writing, erasing, or the like in response to aninstruction from the host apparatus 30. As shown in FIG. 1, thecontroller 20 includes a host interface circuit 21, an internal memory(random access memory: RAM) 22, a processor (central processing unit:CPU) 23, a buffer memory 24, an ECC circuit 25, and a NAND interfacecircuit 26.

The host interface circuit 21 is connected with the host apparatus 30through a host bus and communicates with the host apparatus 30. Forexample, the host interface circuit 21 transfers an instruction and datareceived from the host apparatus 30 to the CPU 23 and the buffer memory24 respectively.

The host interface circuit 21 further transfers data in the buffermemory 24 to the host apparatus 30 in response to an instruction fromthe CPU 23.

The RAM 22 is a semiconductor memory such as a DRAM, and retainsfirmware for managing the semiconductor memory device 10, variousmanagement tables, and the like. The RAM 22 is used as a work area ofthe CPU 23.

The CPU 23 controls the overall operation of the controller 20. Forexample, the CPU 23 issues a write command to the NAND interface circuit26 in response to a write instruction received from the host apparatus30. This operation is similarly executed in reading and erasing. The CPU23 further executes various processes for managing a memory space of thesemiconductor memory device 10, such as wear leveling.

The buffer memory 24 temporarily retains read data received by thecontroller 20 from the semiconductor memory device 10, write datareceived from the host apparatus 30, and the like.

The ECC circuit 25 executes a data error checking and correcting (ECC)process. Specifically, the ECC circuit 25 generates parity bits based onwrite data during data writing. Then, the ECC circuit 25 generatessyndrome bits from the parity bits during data reading, detects anerror, and corrects the detected error.

The NAND interface circuit 26 is connected with the semiconductor memorydevice 10 through a NAND bus and communicates with the semiconductormemory device 10. A signal is transmitted or received between thesemiconductor memory device 10 and the controller 20 in accordance withthe NAND interface. For example, based on the instruction received fromthe CPU 23, the NAND interface circuit 26 transmits a command latchenable signal CLE, an address latch enable signal ALE, a write enablesignal WEn, and a read enable signal REn to the semiconductor memorydevice 10, receives a ready/busy signal RBn from the semiconductormemory device 10, and transmits or receives an input/output signal I/Oto or from the semiconductor memory device 10.

The signals CLE and ALE notifies the semiconductor memory device 10 thatthe input signals I/O transmitted to the semiconductor memory device 10are a command CMD and address information ADD. The signal WEn isasserted at an “L” level and is a signal for causing the semiconductormemory device 10 to take the input signal I/O. The signal REn isasserted at the “L” level and is a signal for reading the output signalI/O from the semiconductor memory device 10.

The ready/busy signal RBn indicates whether the semiconductor memorydevice 10 can receive an instruction from the controller 20. Theready/busy signal RBn is set to an “H” level, when, for example, thesemiconductor memory device 10 is in a ready state in which thesemiconductor memory device 10 can receive the instruction from thecontroller 20, and the ready/busy signal RBn is set to the “L” levelwhen the semiconductor memory device 10 is in a busy state in which thesemiconductor memory device 10 cannot receive the instruction.

The input/output signal I/O is a signal of 8 bits and corresponds to,for example, the command CMD, the address information ADD, data DAT, andthe like. For example, during a write operation, the input/output signalI/O transferred to the semiconductor memory device 10 includes a writecommand CMD issued by the CPU 23 and write data DAT in the buffer memory24. On the other hand, during a read operation, the input/output signalI/O transferred to the semiconductor memory device 10 includes a readcommand, and the input/output signal I/O transferred to the controller20 includes read data DAT.

Examples of the host apparatus 30 using the memory system 1 describedabove include a digital camera and a personal computer.

A semiconductor device may be constituted by combining the semiconductormemory device 10 and the controller 20, for example. Examples of such asemiconductor device include a memory card, such as an SD™ card, and asolid state drive (SSD).

[1-1-2] Configuration of Memory Cell Array 11

Next, a configuration example of the memory cell array 11 will bedescribed with reference to FIG. 2. FIG. 2 shows an example of a circuitconfiguration of the memory cell array 11 and the sense amplifier module17, and shows a detailed circuit configuration of one block BLK in thememory cell array 11. As shown in FIG. 2, the block BLK includes aplurality of NAND strings NS.

The NAND strings NS are provided to respective bit lines BL0 to BL(m−1)((m−1) is an integer of 1 or more), and each include, for example, eightmemory cell transistors MT (MT0 to MT7) and select transistors ST1 andST2. The number of the memory cell transistors MT included in one NANDstring NS is not limited thereto, and may be a given number.

The memory cell transistor MT includes a control gate and a chargestorage layer, and nonvolatilely retains data. The memory celltransistors MT0 to MT7 are connected in series between a source of theselect transistor ST1 and a drain of the select transistor ST2. Controlgates of the memory cell transistors MT0 to MT7 in the same block BLKare commonly connected to word lines WL0 to WL7 respectively.

Control gates of the select transistors ST1 and ST2 in the same blockBLK are commonly connected to select gate lines SGD and SGSrespectively. Drains of the select transistors ST1 on the same column inthe blocks BLK are commonly connected to the corresponding bit line BL.That is, the bit line BL commonly connects the NAND strings NS on thesame column in a plurality of blocks BLK. Sources of the selecttransistor ST2 in the blocks BLK are commonly connected to a source lineSL.

In the above structure, a collection of 1-bit data retained in thememory cell transistors MT connected to the common word line WL isreferred to as a “page”. Accordingly, when the TLC method for storing3-bit data in one memory cell transistor MT is used, data of three pagesis stored in a collection of memory cell transistors MT connected to theword line WL. The semiconductor memory device 10 may execute writing andreading of data for each page, or for each word line WL.

The distribution of the threshold voltages of the memory celltransistors MT described above is as shown in, for example, FIG. 3. FIG.3 shows an example of threshold distribution of memory cell transistorsMT, assigned data, and voltages used in the write and read operations.The vertical axis and the horizontal axis in FIG. 3 correspond to thenumber of memory cell transistors MT and the threshold voltage Vthrespectively. In the following description, 3-bit data retained in onememory cell transistor MT is referred to as a lower bit, a middle bit,and an upper bit in order from a lower bit. A collection of lower bitsretained in the memory cell transistors MT connected to the same wordline WL is referred to as a “lower page”, and a collection of middlebits is referred to as a “middle page”, and a collection of upper bitsis referred to as an “upper page”.

As shown in FIG. 3, when data is written by the TLC method, thethreshold distribution of the memory cell transistors MT is divided intoeight. The eight threshold distributions are referred to as an “ER”level, an “A” level, a “B” level, a “C” level, a “D” level, an “E”level, an “F” level, and a “G” level in order from a lower thresholdvoltage.

The “ER” level corresponds to an erase state of the memory celltransistor MT. The “A” level, the “B” level, the “C” level, the “D”level, the “E” level, the “F” level, and the “G” level correspond towrite states of the memory cell transistor MT, and are formed by writeoperations using verify voltages AV, BV, CV, DV, EV, FV, and GVrespectively. The relation between these voltage values is as follows:AV<BV<CV<DV<EV<FV<GV

Specifically, the threshold voltage of the memory cell transistor MTincluded in the “ER” level is lower than the voltage AV. The thresholdvoltage of the memory cell transistor MT included in the “A” level isequal to or higher than the voltage AV, and lower than the voltage BV.The threshold voltage of the memory cell transistor MT included in the“B” level is equal to or higher than the voltage BV and lower than thevoltage CV. The threshold voltage of the memory cell transistor MTincluded in the “C” level is equal to or higher than the voltage CV andlower than the voltage DV. The threshold voltage of the memory celltransistor MT included in the “D” level is equal to or higher than thevoltage DV and lower than the voltage EV. The threshold voltage of thememory cell transistor MT included in the “E” level is equal to orhigher than the voltage EV and lower than the voltage FV. The thresholdvoltage of the memory cell transistor MT included in the “F” level isequal to or higher than the voltage FV and lower than the voltage GV.The threshold voltage of the memory cell transistor MT included in the“G” level is equal to or higher than the voltage GV.

In the present embodiment, data is assigned to the memory celltransistors MT included in each threshold distribution as follows:

“ER” level: “111” (“lower bit/middle bit/upper bit”) data,

“A” level: “101” data,

“B” level: “001” data,

“C” level: “000” data,

“D” level: “100” data,

“E” level: “110” data,

“F” level: “010” data, and

“G” level: “011” data.

In the threshold distributions described above, the read voltages areset between respective adjacent threshold distributions. For example, aread voltage AR for determining whether a memory cell transistor MT hasa threshold voltage of the “ER” level or a threshold voltage of the “A”level or higher is set between the maximum threshold voltage at the “ER”level and the minimum threshold voltage at the “A” level. A read voltageBR for determining whether a memory cell transistor MT has a thresholdvoltage of the “A” level or a threshold voltage of the “B” level orhigher is set between the maximum threshold voltage at the “A” level andthe minimum threshold voltage at the “B” Level. Other read voltages CR,DR, ER, FR, and GR are also set similarly to the read voltages AR andBR. Then, a read pass voltage Vread is set to a voltage higher than themaximum threshold voltage in the threshold distribution at the “G”level. The voltage Vread is a voltage applied to the unselected wordline during a read operation, and the memory cell transistor MT in whichthe voltage Vread is applied to the control gate is turned ON regardlessof the retained data.

In the read operation, the lower page data is determined by the readresults using the voltages BR, DR, and FR. The middle page data isdetermined by the read results using the voltages AR and ER. The upperpage data is determined by the read results using the voltages CR andGR. That is, the lower page data, the middle page data, and the upperpage data are determined by executing the read operation three times,twice, and twice respectively. Hereinafter, such data assignment isreferred to as a “3-2-2 code”.

As various verify voltages and various read voltages set betweenrespective threshold distributions, the same voltage value or adifferent voltage value may be set.

[1-1-3] Configuration of Sense Amplifier Module 17

Returning to FIG. 2, a configuration example of the sense amplifiermodule 17 will be described. As shown in FIG. 2, the sense amplifiermodule 17 includes sense amplifier units SAU (SAU0 to SAU (m−1))provided for each bit line BL.

The sense amplifier units SAU each include a sense amplifier part SA,latch circuits ADL, BDL, CDL, and XDL, and an operation unit OP. Thesense amplifier part SA, the latch circuits ADL, BDL, CDL, and XDL, andthe operation unit OP are connected so as to be able to transmit andreceive data to and from each other.

The sense amplifier part SA senses read data on the corresponding bitline BL during a read operation, and determines whether the read data is“0” or “1”. The sense amplifier part SA further applies a voltage to thebit line BL during a write operation based on write data.

The latch circuits ADL, BDL, and CDL temporarily retain read data andwrite data. The read data determined by the sense amplifier part SAduring the read operation and the write data transferred to the latchcircuit XDL during the write operation are transferred to, for example,any one of the latch circuits ADL, BDL, and CDL.

The operation unit OP can perform various operations, such as a logicalsum (OR) operation, logical product (AND) operation, and exclusivelogical sum (XOR) operation, with respect to data retained in the latchcircuits ADL, BDL, and CDL.

The latch circuit XDL is used for inputting and outputting data betweenthe sense amplifier unit SAU and the controller 20. For example, thedata received from the controller 20 is transferred to the latch circuitADL, BDL, or CDL, or to the sense amplifier part SA through the latchcircuit XDL. Similarly, data retained in the latch circuit ADL, BDL, orCDL, or the sense amplifier part SA is transferred to the controller 20through the latch circuit XDL.

The latch circuit XDL functions as a cache memory of the semiconductormemory device 10. For example, if the latch circuits ADL, BDL, and CDLare being used, the semiconductor memory device 10 can be in the readystate as long as the latch circuit XDL is not used.

FIG. 4 shows a detailed circuit configuration of the sense amplifierpart SA and the latch circuit ADL described above. As shown in FIG. 4,the sense amplifier part SA includes a p-channel MOS transistor 40,n-channel MOS transistors 41 to 47, and a capacitor 48. The latchcircuit ADL includes inverters 50 and 51 and n-channel MOS transistors52 and 53.

In the transistor 40, one end is connected to a power supply terminal,and the gate is connected to a node INV. In the transistor 41, one endis connected to the other end of the transistor 40, the other end isconnected to a node COM, and a control signal BLX is input to the gate.In the transistor 42, one end is connected to the node COM, the otherend is connected to the corresponding bit line BL, and a control signalBLC is input to the gate. In the transistor 43, one end is connected tothe node COM, the other end is connected to a node SRC, and the gate isconnected to a node INV. In the transistor 44, one end is connected tothe other end of the transistor 40, the other end is connected to a nodeSEN, and a control signal HLL is input to the gate. In the transistor45, one end is connected to the node SEN, the other end is connected tothe node COM, and a control signal XXL is input to the gate. In thetransistor 46, one end is connected to the ground terminal, and the gateis connected to the node SEN. In the transistor 47, one end is connectedto the other end of the transistor 46, the other end is connected to abus LBUS, and a control signal STB is input to the gate. In thecapacitor 48, one end is connected to the node SEN, and a clock CLK isinput to the other end. A voltage Vdd which is, for example, a powersupply voltage of the semiconductor memory device 10 is applied to thepower supply terminal connected to the one end of the transistor 40. Avoltage Vss which is, for example, a ground voltage of the semiconductormemory device 10 is applied to the node SRC.

In the inverter 50, the input terminal is connected to a node LAT, andthe output terminal is connected to a node INV. In the inverter 51, theinput terminal is connected to the node INV and the output terminal isconnected to the node LAT. In the transistor 52, one end is connected tothe node INV, the other end is connected to the bus LBUS, and a controlsignal STI is input to the gate. In the transistor 53, one end isconnected to the node LAT, the other end is connected to the bus LBUS,and a control signal STL is input to the gate. Since the circuitconfiguration of the latch circuits BDL and CDL is similar to thecircuit configuration of the latch circuit ADL described above, thedescription thereof is omitted.

In the configuration of the sense amplifier unit SAU described above,various control signals are generated by, for example, the sequencer 14.In the read operation, the timing of when the data read by the senseamplifier part SA is determined is based on the timing of when thesignal STB is asserted. In various operations, the transistor 42 clampsthe voltage of the bit line BL based on the signal BLC.

The configuration of the sense amplifier module 17 is not limitedthereto and may be variously changed. For example, the number of latchcircuits included in the sense amplifier unit SAU is not limitedthereto, and is designed based on the number of bits of data retained inone memory cell transistor MT.

[1-2] Write Operation of Memory System 1

Next, the write operation of the memory system 1 will be described. Inthe first embodiment, the memory system 1 executes a two-stage writeoperation when storing 3-bit data in the memory cell transistor MT bythe TLC method. In the following description, the first-stage writeoperation of the two-stage write operation is referred to as a firstwrite operation, and the second-stage write operation is referred to asa second write operation.

In the first write operation, the controller 20 transmits lower pagedata to the semiconductor memory device 10, and the semiconductor memorydevice 10 writes the lower page data received from the controller 20 tothe memory cell array 11.

In the second write operation, the controller 20 transmits middle pagedata and upper page data to the semiconductor memory device 10, and thesemiconductor memory device 10 writes, based on the middle page data andthe upper page data received from the controller 20 and the lower pagedata read from the memory cell array 11, 3-page data to the memory celltransistor MT from which the lower page data has been read.

Hereinafter, the details of the write operation of the memory system 1will be described with reference to FIGS. 5 and 6. FIG. 5 is a flowchartshowing an example of the write operation of the memory system 1, andFIG. 6 shows a command sequence of the operation corresponding to FIG.5. It is assumed that the controller 20 receives the write data from thehost apparatus 30 and stores the received write data in the RAM 22 in apage unit before the write operation described below is executed. In thefollowing description, it is assumed that a command CMD received by thesemiconductor memory device 10 is stored in the command register 12, andaddress information ADD received by the semiconductor memory device 10is stored in the address register 13.

As shown in FIG. 5, the controller 20 first issues a first command setincluding address information ADD specifying the word line WL0 andtransmits the first command set to the semiconductor memory device 10(step S10). Specifically, as shown in FIG. 6, the controller 20 firstissues a command “80h” and transmits the command “80h” to thesemiconductor memory device 10. The command “80h” corresponds to anaddress and data input reception command for writing, and is a commandinstructing the semiconductor memory device 10 to write data. Next, thecontroller 20 continuously transmits the address information ADDspecifying the word line WL0 and the lower page data DAT to thesemiconductor memory device 10. The semiconductor memory device 10allows the received data DAT to be retained in the latch circuit XDL ofthe sense amplifier module 17. Then, the controller 20 issues a command“10h” and transmits the command “10h” to the semiconductor memory device10. The command “10h” is a command instructing the semiconductor memorydevice 10 to execute the write operation. This series of commandsequences corresponds to the first command set.

When the command “10h” is stored in the command register 12, thesequencer 14 sets the ready/busy signal RBn to the “L” level. Then, thesequencer 14 transfers the write data retained in the latch circuit XDLto, for example, the latch circuit ADL, and executes the first writeoperation in which the word line WL0 is selected (step S11).

Here, an example of a change in the threshold distribution of the memorycell transistors MT by the first write operation will be described withreference to FIG. 7. FIG. 7 shows 1-page data used in the first writeoperation and threshold distributions of the memory cell transistors MTbefore and after the first write operation is executed. In the followingdescription, it is assumed that an increase in the threshold voltage ofa write-inhibited memory cell transistor MT and a memory cell transistorMT having passed a verify is suppressed by applying channel boost or thelike.

As shown in FIG. 7, the threshold voltages of the memory celltransistors MT before the execution of the first write operation aredistributed to the “ER” level, which is the erase state. In the firstwrite operation, the semiconductor memory device 10 executes the writeoperation for 1-page data based on the lower page data input from thecontroller 20, and forms two threshold distributions from the thresholddistribution at the “ER” level.

Specifically, the semiconductor memory device 10 sets the memory celltransistors MT to which “1” (“lower bit”) data is written as a writeinhibit state, and executes the write operation using a voltage M1V as averify voltage to the memory cell transistors MT to which “0” data iswritten. The voltage M1V is lower than the voltage CR. Thus, thethreshold voltages of the memory cell transistors MT to which “1” datais written are distributed to the “ER” level and the threshold voltagesof the memory cell transistors MT to which “0” data a written aredistributed to the “M1” level.

The threshold voltage of the memory cell transistor MT included in the“M1” level is equal to or higher than the voltage M1V and lower than thevoltage CV. That is, in consideration that the threshold distribution atthe “M1” level changes to the threshold distribution at the “B” level orhigher by the second write operation, the verify voltage M1V is set sothat the threshold voltage of the memory cell transistor MT havingpassed the verify is to be lower than the voltage CR.

Returning to FIG. 5, when the first write operation in step S11 isended, the sequencer 14 sets the ready/busy signal RBn to the “H” level(step S12). When detecting that the semiconductor memory device 10 is inthe ready state, the controller 20 issues a first command set includingthe address information ADD specifying the word line WL1, and transmitsthe first command set to the semiconductor memory device 10 (step S13).When receiving the first command set from the controller 20, thesemiconductor memory device 10 sets the ready/busy signal RBn to the “L”level and executes the first write operation in which the word line WL1is selected (step S14). Thus, the lower page data is written to thememory cell transistors MT connected to the word line WL1.

When the first write operation in step S14 is ended, the sequencer 14sets the ready/busy signal RBn to the “H” level (step S15). Whendetecting that the semiconductor memory device 10 is in the ready state,the controller 20 issues a second command set including the addressinformation ADD specifying the word line WL0 and transmits the secondcommand set to the semiconductor memory device 10 (step S16).

Specifically, as shown in FIG. 6, the controller 20 first issues acommand “80h” and transmits the command “80h” to the semiconductormemory device 10. Then, the controller 20 continuously transmits theaddress information ADD specifying the word line WL0 and the middle pagedata DAT to the semiconductor memory device 10. Then, the semiconductormemory device 10 allows the received data DAT to be retained in thelatch circuit XDL of the sense amplifier module 17. Next, the controller20 issues a command “xxh” and transmits the command “xxh” to thesemiconductor memory device 10. The command “xxh” is a commandindicating that the data received so far corresponds to 1-page data.When the command “xxh” is stored in a command register 12, the sequencer14 sets the ready/busy signal RBn to the “L” level, and transfers thewrite data retained in the latch circuit XDL to, for example, the latchcircuit ADL. The sequencer 14 sets the ready/busy signal RBn to the “H”level. This operation is indicated as “dummy busy” in FIG. 6. When theready/busy signal RBn is set to the “H” level, the controller 20 issuesa command “80h” and transmits the command “80h” to the semiconductormemory device 10. Then, the controller 20 continuously transmits theaddress information ADD specifying the word line WL0 and the upper pagedata DAT to the semiconductor memory device 10. The semiconductor memorydevice 10 then allows the received data DAT to be retained in the latchcircuit XDL of the sense amplifier module 17. Next, the controller 20issues a command “10h” and transmits the command “10h” to thesemiconductor memory device 10. This series of command sequencescorresponds to the second command set.

When the command “10h” is stored in the command register 12, thesequencer 14 sets the ready/busy signal RBn to the “L” level. Then, thesequencer 14 transfers the write data retained in the latch circuit XDLto, for example, the latch circuit BDL, and executes the second writeoperation in which the word line WL0 is selected (step S17).

Here, an example of a change in the threshold distribution of the memorycell transistors MT by the second write operation will be described withreference to FIG. 8. FIG. 8 shows 3-page data used in the second writeoperation and threshold distributions of the memory cell transistors MTbefore and after the second write operation is executed.

As shown in FIG. 8, the threshold voltages of the memory celltransistors MT before the execution of the second write operation aredistributed to the “ER” level and the “M1” level. In the second writeoperation, the semiconductor memory device 10 writes, based on themiddle page data and the upper page data input from the controller 20and the lower page data read from the memory cell array 11, 3-page datato the memory cell transistor MT from which the lower page data is read,and forms eight threshold distributions from the threshold distributionsat the “ER” level and “M1” level.

Specifically, the sequencer 14 first executes an internal data load(IDL). In the IDL, before a program pulse (program voltage) is appliedto a selected word line WL, there is a data read operation of the datastored in, for example, the memory cell transistor MT corresponding tothe applicable word line WL. In the IDL in the present embodiment, aread operation using a voltage M1R is executed. The voltage M1R is setbetween the threshold distribution at the “ER” level and the thresholddistribution at the “M1” level. The sense amplifier part SA determineswhether the threshold voltage of the memory cell transistor MT is lowerthan the voltage M1R by the read operation using the voltage M1R, andtransfers the determined data to, for example, the latch circuit CDL. Inthis manner, the lower page data (“1” data or “0” data) written by thefirst write operation is restored to the latch circuit in the senseamplifier unit SAU.

The sequencer 14 sets the memory cell transistor MT to which “111”(“lower bit/middle bit/upper bit”) data is written as a write inhibitstate, and executes the write operations using the voltages AV, BV, CV,DV, EV, FV, and GV as verify voltages to the memory cell transistors MTto which “101” data, “001” data, “000” data, “100” data, “110” data,“010” data, and “011” data are written respectively. Thus, the thresholddistributions at the “A” level, the “D” level, and the “E” level areformed from the threshold distribution at the “ER” level, and thethreshold distributions at the “B” level, the “C” level, the “F” level,and the “G” level are formed from the threshold distribution at the “M1”level.

Returning to FIG. 5, when the second write operation in step S17 isended, the sequencer 14 sets the ready/busy signal RBn to the “H” level(step S18). When detecting that the semiconductor memory device 10 is inthe ready state, the controller 20 issues a first command set includingthe address information ADD specifying the word line WL2 and transmitsthe first command set to the semiconductor memory device 10 (step S19).When receiving the first command set from the controller 20, thesemiconductor memory device 10 sets the ready/busy signal RBn to the “L”level and executes the first write operation in which the word line WL2is selected (step S20). Thus, the lower page data is written to thememory cell transistors MT connected to the word line WL2.

When the second write operation in step S20 is ended, the sequencer 14sets the ready/busy signal RBn to the “H” level (step S21). Whendetecting that the semiconductor memory device 10 is in the ready state,the controller 20 issues a second command set specifying the word lineWL1 and transmits the second command set to the semiconductor memorydevice 10 (step S22). When receiving the second command set from thecontroller 20, the semiconductor memory device 10 sets the ready/busysignal RBn to the “L” level and executes the second write operation inwhich the word line WL2 is selected (step S23). Thus, the 3 page data iswritten to the memory cell transistors MT connected to the word lineWL1. When the second write operation in step S23 is ended, the sequencer14 sets the ready/busy signal RBn to the “H” level (step S24).

In the subsequent write operation, an operation similar to steps S19 toS24 is repeated. When the second write operation corresponding to thelast 3-page data is ended, the memory system 1 ends the write operation.In FIG. 5, an example has been illustrated in which the write operation(for example, after steps S11, S14, S17, S20, and S23) is executed, andthen the next command set and data are input after the ready/busy signalRBn becomes the “H” level, but the operation order is not limitedthereto. For example, by having an extra number of latch circuits (forexample, latch circuits XDL) illustrated with reference to FIG. 2 forwrite cache usage, it is possible to input the next command set and dataduring the write operation.

In the first and second write operations described above, the waveformof the voltage applied to the selected word line WL is as shown in, forexample, FIG. 9. FIG. 9 shows an example of an input/output signal I/Oand a voltage applied to a selected word line WL. In the followingdescription, the selected word line WL is referred to as a selected wordline WL_sel.

As shown in FIG. 9, when receiving the first and second command sets,the semiconductor memory device 10 is in a busy state and executes thefirst and second write operations.

In the first write operation, first, the row decoder 16 applies avoltage Vpgm1 to the selected word line WL_sel. The voltage Vpgm1 is aprogram voltage and is a high voltage capable of injecting electronsinto a charge storage layer of the memory cell transistor MT. When thevoltage Vpgm1 is applied to the selected word line WL_sel, electrons areinjected into the charge storage layer by the potential differencebetween the gate and the channel, and the threshold voltage of thecorresponding memory cell transistor MT is increased. In awrite-inhibited memory cell transistor MT set as a write inhibit statein the memory cell transistors MT connected to the selected word lineWL_sel, variation of the threshold voltage is suppressed by, forexample, boosting the channel of the corresponding NAND string NS toreduce the potential difference between the gate and the channel in thememory cell transistors MT connected to the selected word line WL_sel.Next, the row decoder 16 applies a voltage Vvfy, and the sense amplifiermodule 17 determines whether or not the threshold voltage of a writingtarget memory cell transistor MT exceeds the voltage Vvfy. The voltageVvfy is a verify voltage and is, for example, the voltage M1V shown inFIG. 7.

The operation of applying the program voltage and the verify voltagedescribed above corresponds to one program loop. The sequencer 14repeats such a program loop while increasing a value of the programvoltage in increments of ΔVpgm1. Then, when the memory cell transistorMT passes the verify using, for example, the voltage M1V by repeatingthe program loop, the sequencer 14 ends the first write operation, andthe semiconductor memory device 10 changes to the ready state.

In the second write operation, first, the row decoder 16 applies thevoltage M1R to the selected word line WL. This operation corresponds tothe IDL, and the sense amplifier module 17 reads 1-page data (lower pagedata) stored in the memory cell transistors MT connected to the selectedword line WL_sel. Subsequently, the sequencer 14 repeats the programloop based on the 3-page data retained in the latch circuits ADL, BDL,and CDL. Note that, in the program loop in the second write operation,the value of the program voltage to be applied first, the value of theprogram voltage incremented for each program loop, and the verifyvoltage to be used are different from those in the program loop in thefirst write operation.

Specifically, the value of the program voltage to be applied first isVpgm2, and the value of the incremented program voltage is ΔVpgm2. Asthe verify voltage Vvfy, several voltages are selected and used from thevoltages AV, BV, CV, DV, EV, FV, and GV in ascending order of thevoltage value. Vpgm2 is smaller than Vpgm1, and ΔVpgm2 is smaller thanΔVpgm1. As described above, in the second write operation, the thresholdvoltage of the memory cell transistor MT is finely controlled by usingthe program voltage and ΔVpgm smaller than those in the first writeoperation. When the memory cell transistor MT passes the verify using,for example, the voltage GV by repeating the program loop, the sequencer14 ends the second write operation, and the semiconductor memory device10 changes to the ready state.

[1-3] Effects of First Embodiment

The memory system 1 according to the first embodiment described abovecan improve reliability of written data. Hereinafter, the details of theeffects of the first embodiment will be described.

In a semiconductor memory device, a threshold voltage of a memory celladjusted to a desired threshold voltage by a write operation can varyafter a write operation to the memory cell. For example, in a memorycell using a NOMOS film, a phenomenon called an initial fall occurs inwhich a certain amount of electrons escape from a charge storage layerand the threshold voltage falls after the electrons are injected intothe charge storage layer of the memory cell by a write operation. Thevariation amount of the threshold voltage due to the initial fall isbased on the amount of electrons injected into the charge storage layerof the memory cell by the write operation.

In addition, when the write operation is executed to the memory celladjacent to the memory cell to which data is written, the thresholdvoltage of the adjacent memory cell is increased, and the thresholdvoltage of the memory cell to which the data has already been written isincreased accordingly. This phenomenon is caused by a change in theparasitic capacitance between adjacent memory cells, and the variationamount in the threshold voltage in the memory cell is increased as thevariation amount of the threshold voltage in the adjacent memory cell isincreased.

If the threshold voltage of the memory cell varies due to the initialfall of the threshold distribution or the influence of the parasiticcapacitance between adjacent memory cells, the threshold distribution ofthe memory cells can be expanded, and the number of error bits duringthe read operation can be increased accordingly.

For this reason, the memory system 1 according to the first embodimentapplies a two-stage write operation when writing 3-page data by the TLCmethod. Specifically, the semiconductor memory device 10 writes 1-pagedata including a lower bit in the first-stage write operation (firstwrite operation), and 2-page data including a middle bit and an upperbit in the subsequent second-stage write operation (second writeoperation). In the memory system 1 according to the first embodiment,the first write operation in which the adjacent word line WL is selectedis executed between the first write operation and the second writeoperation. Specifically, when, for example, the first write operation inwhich the word line WL0 is selected, the first write operation in whichthe adjacent word line WL1 is selected is executed next, and then thesecond write operation in which the word line WL0 is selected isexecuted.

In this case, in the memory cells corresponding to the word line WL0,the initial fall occurs while the first write operation in which theword line WL1 is selected is being executed. Then, the second writeoperation, in which the word line WL0 is selected, is executed from astate of being influenced by the change in the parasitic capacitancebetween the adjacent memory cells caused by the first write operation inwhich the word line WL1 is selected. As a result, in the thresholddistribution finally obtained, these influences can be ignored.

Furthermore, the second write operation is to be a write operation tothe memory cell transistor MT having the threshold voltage which hasbeen increased to some extent by the first write operation. As a result,the variation amount of the threshold voltage of the memory celltransistor MT caused by the second write operation becomes small. Thatis, the amount of electrons to be injected into the charge storage layerof the memory cell transistor MT in the second write operation issmaller than that in the case where data is written in three bits at atime.

Thus, the memory system 1 according to the first embodiment can suppressthe influences of the initial fall of the threshold voltage and theparasitic capacitance between the adjacent memory cells which occurafter data is written. Accordingly, the memory system 1 according to thepresent embodiment can suppress the expansion of the thresholddistribution in the writing operation, and improve the reliability ofthe written data.

The first write operation described above is a write operation usingonly lower page data. The threshold distribution after the first writeoperation is only required to be roughly formed, because the thresholddistribution is finely formed by the following second write operation.In the first write operation, it is possible to set the initial value ofthe program voltage Vpgm used for the write operation and ΔVpgm for eachprogram loop so as to be larger than those in the second writeoperation. Thus, the memory system 1 according to the first embodimentcan accelerate the first-stage write operation when executing thetwo-stage write operation.

In addition, since the data written in the memory cell transistor MT bythe first write operation appears to be binary, it is possible toexecute the read operation for the lower page data. For this reason, thememory system 1 according to the first embodiment restores the lowerpage data to be used in the second write operation by reading the datafrom the corresponding memory cell by the IDL. Thus, when 3-page data iswritten in the first and second write operations, the controller 20transmits 1-page data to be used in the first write operation to thesemiconductor memory device 10 and then can discard the data.Accordingly, the memory system 1 according to the first embodiment cansuppress the storage capacity of the RAM 22 and buffer memory 24, andsuppress the circuit area of the controller 20.

Furthermore, the memory system 1 according to the first embodimentrestores the lower page data by the IDL, and can omit data input for onepage in the second write operation. Accordingly, the memory system 1according to the first embodiment can shorten the time of data input inthe second write operation, and accelerate the write operation.

In the memory system 1 according to the first embodiment, the 3-2-2 codedescribed with reference to FIG. 3 is applied as the assignment of datato be written to the memory cell transistor MT. In the 3-2-2 code, thereading at the voltages AR and GR at which error bits are easilygenerated is assigned to the read operation for the middle page data andthe upper page data, which are determined by executing the readingtwice. In the read operation for the lower page data, by determining thedata by reading at the voltages BR, DR, and FR at which error bits arerelatively hard to generate, an increase in the number of error bits dueto the increase in the number of read times is suppressed. Thus, thememory system 1 can disperse the number of error bits generated in theread operations for the lower page data, the middle page data, and theupper page data in the 3-page data read operation, and enhance thepossibility of succeeding in the error correction by the ECC circuit 25.Accordingly, the memory system 1 according to the present embodiment canimprove the reliability of the read operation.

[2] Second Embodiment

Next, a memory system 1 according to a second embodiment will bedescribed. In the memory system 1 according to the second embodiment,data assignment different from that in the memory system 1 described inthe first embodiment is applied, and 2-page data is written in a firstwrite operation. Hereinafter, differences between the memory system 1according to the second embodiment and that according to the firstembodiment will be described.

[2-1] Data Assignment of Memory Cell

First, data assignment to be applied to the memory system 1 according tothe second embodiment will be described with reference to FIG. 10. FIG.10 shows threshold distribution of memory cell transistors MT, assigneddata, and voltages used in write and read operations, and dataassignment is different from that shown in FIG. 3 described in the firstembodiment.

As shown in FIG. 10, in the present embodiment, data is assigned to thememory cell transistors MT included in each threshold distribution asfollows:

“ER” level: “111” (“lower bit/middle bit/upper bit”) data,

“A” level: “110” data,

“B” level: “100” data,

“C” level: “101” data,

“D” level: “001” data,

“E” level: “011” data,

“F” level: “010” data, and

“G” level: “000” data.

In the read operation, the lower page data is determined by the readresult using a voltage DR. The middle page data is determined by theread results using voltages BR, ER, and GR. The upper page data isdetermined by the read results using voltages AR, CR, and FR. That is,the lower page data, the middle page data, and the upper page data aredetermined by executing the read operation once, three times, and threetimes respectively. Hereinafter, such data assignment is referred to asa “1-3-3 code”.

[2-2] Write Operation of Memory System 1

Next, a write operation of the memory system 1 according to the secondembodiment will be described with reference to FIGS. 11 and 12. FIG. 11is a flowchart showing the write operation of the memory system 1, andFIG. 12 shows a command sequence of the operation corresponding to FIG.11. Steps S30 to S44 shown in FIGS. 11 and 12 correspond to steps S10 toS24 respectively described in the first embodiment with reference toFIGS. 5 and 6. In the second embodiment, details of a first command set,a first write operation, and a second write operation are different fromthose in the first embodiment.

First, the details of the first command set in the second embodimentwill be described. As shown in FIG. 12, the first command set in thesecond embodiment is similar to that of the write data DAT in which asecond command set described in the first embodiment with reference toFIG. 6 is converted from the middle and upper page data to the lower andmiddle page data.

Next, the details of the first write operation in the second embodimentwill be described with reference to FIG. 13. FIG. 13 shows 2-page dataused in the first write operation and threshold distributions of thememory cell transistors MT before and after the first write operation isexecuted. In the first write operation in the second embodiment, thewrite operation for 2-page data is executed based on the first commandset.

As shown in FIG. 13, the threshold voltages of the memory celltransistors MT before the execution of the first write operation aredistributed to the “ER” level, which is an erase state. In the firstwrite operation, a semiconductor memory device 10 executes the writeoperation for 2-page data based on the lower page data and the middlepage data input from a controller 20, and forms four thresholddistributions from the threshold distribution at the “ER” level.

Specifically, a sequencer 14 sets the memory cell transistors MT towhich “11” (“lower bit/middle bit”) data is written as a write inhibitstate, and executes the write operations using voltages M1V, M2V, andM3V as verify voltages to the memory cell transistors MT to which “10”data, “00” data, and “01” data are respectively written. The voltage M1Vis lower than the voltage CR. The voltage M2V is higher than the voltageM1V and lower than the voltage ER. The voltage M3V is higher than thevoltage M2V and lower than the voltage FR. Thus, the threshold voltagesof the memory cell transistors MT to which the “11” data is written aredistributed to the “ER” level, the threshold voltages of the memory celltransistors MT to which the “10” data is written are distributed to the“M1” level, the threshold voltages of the memory cell transistors MT towhich the “00” data is written are distributed to the “M2” level, andthe threshold voltages of the memory cell transistors MT to which the“01” data is written are distributed to the “M3” level.

The threshold voltage of the memory cell transistor MT included in the“M1” level is equal to or higher than the voltage M1V and lower than thevoltage CR, the threshold voltage of the memory cell transistor MTincluded in the “M2” level is equal to or higher than the voltage M2Vand lower than the voltage ER, and the threshold voltage of the memorycell transistor MT included in the “M3” level is equal to or higher thanthe voltage M3V and lower than the voltage FR. The verify voltages M1V,M2V, and M3V are set so that the threshold voltages of the memory celltransistors MT having passed the verify do not exceed the voltages CR,ER, and FR respectively. The verify voltages M1V and M2V may be set sothat the interval between the threshold distribution at the “M1” leveland the threshold distribution at the “M2” level is wider than theintervals between other threshold distributions.

Next, the details of the second write operation in the second embodimentwill be described with reference to FIG. 14. FIG. 14 shows 3-page dataused in the second write operation and threshold distributions of thememory cell transistors MT before and after the second write operationis executed. In the second write operation in the present embodiment,the write operation for 3-page data using the IDL is executed based onthe second command set similar to that in the first embodiment.

As shown in FIG. 14, the threshold voltages of the memory celltransistors MT before the execution of the second write operation aredistributed to the “ER” level, the “M1” level, the “M2” level, and the“M3” level. In the second write operation, the semiconductor memorydevice 10 executes the write operation for 3-page data based on themiddle and upper page data input from the controller 20 and the lowerpage data read from a memory cell array 11, and forms eight thresholddistributions from the threshold distributions at the “ER” level, the“M1” level, the “M2” level, and the “M3” level.

Specifically, the sequencer 14 first executes an internal data load(IDL). In the IDL in the present embodiment, a read operation using thevoltage M2R is executed. The voltage M2R is a voltage set between thethreshold distribution at the “M1” level and the threshold distributionat the “M2” level. Thus, since the lower page data at the “ER” level andthe “M1” level is “1”, and the lower page data at the “M2” level and the“M3” level is “0”, a sense amplifier module 17 can determine whether thelower page data is “1” or “0” by determining whether the thresholdvoltage of the memory cell transistor MT is lower than the voltage M2R.In this manner, the semiconductor memory device 10 restores the “1” dataand “0” data (lower page data) written by the first write operation to alatch circuit in a sense amplifier unit SAU.

The sequencer 14 sets the memory cell transistors MT to which “111”(“lower bit/middle bit/upper bit”) data is written as a write inhibitstate, and executes the write operations using the voltages AV, BV, CV,DV, EV, FV, and GV as verify voltages to the memory cell transistors MTto which the “110” data, the “100” data, the “101” data, the “001” data,the “011” data, the “010” data, and the “000” data are respectivelywritten. Thus, the threshold distribution at the “A” level is formedfrom the threshold distribution at the “ER” level, the thresholddistributions at the “B” level and “C” level are formed from thethreshold distribution at the “M1” level, the threshold distributions atthe “D” level and the “G” level are formed from the thresholddistribution at the “M2” level, and the threshold distributions at the“E” level and “F” level are formed from the threshold distribution atthe “M3” level.

[2-3] Effects of Second Embodiment

As described above, the memory system 1 according to the secondembodiment executes the two-stage write operation using the 1-3-3 code,and writes 2-page data (the lower page data and the middle page data) inthe first write operation.

In this case, in the first write operation in the second embodiment,data can be written to the memory cell transistor MT to a higherthreshold voltage, as compared with the first write operation in thefirst embodiment. In the second write operation in the presentembodiment, since the write operation starts from a threshold voltagehigher than the second write operation in the first embodiment, it ispossible to further suppress the variation amount of the thresholdvoltage, as compared with the second write operation in the firstembodiment.

Therefore, the memory system 1 according to the second embodiment canfurther suppress the influences of the initial fall of the thresholdvoltage and the parasitic capacitance between the adjacent memory cellsthat occur after data is written, as compared with the first embodiment.Accordingly, the memory system 1 according to the second embodiment canfurther suppress the expansion of the threshold distribution in thewriting operation as compared with the first embodiment, and improve thereliability of the written data.

In the memory system 1 according to the second embodiment, the 1-3-3code described with reference to FIG. 10 is applied as the assignment ofdata to be written to the memory cell transistor MT. In the 1-3-3 code,the lower page data can be read in a state of writing 2-page dataconstituted by the lower page and the middle page. That is, the memorysystem 1 according to the second embodiment can restore the lower pagedata by the IDL, similar to the first embodiment.

Due to this, the memory system 1 according to the second embodiment canomit data input for one page in the second write operation. Accordingly,the memory system 1 according to the second embodiment can shorten thetime of data input in the second write operation, similar to the firstembodiment, and can accelerate the write operation. In addition, sincethe lower page data used in the second write operation is restored bythe IDL, the controller 20 can discard the lower page data used in thefirst write operation after transmitting the lower page data to thesemiconductor memory device 10. Accordingly, the memory system 1according to the second embodiment can control the storage capacity of aRAM 22 and a buffer memory 24, and control the circuit area of thecontroller 20.

In the second embodiment, the sequencer 14 controls, in the first writeoperation, the interval formed between the threshold distribution at the“M1” level and the threshold distribution at the “M2” level in the fourthreshold distributions so as to be wider than the intervals betweenother threshold distributions. The read margin in the IDL in the secondwrite operation is thereby widened, and it is possible to suppress thenumber of read error bits by the IDL.

Accordingly, the memory system 1 according to the second embodiment cansuppress the deterioration of the reliability of the data to be writtenin the second write operation.

[3] Third Embodiment

Next, a memory system 1 according to a third embodiment will bedescribed. In the memory system 1 according to the third embodiment,data assignment different from that in the memory system 1 described inthe second embodiment is applied. Hereinafter, differences between thememory system 1 according to the third embodiment and that according tothe first and second embodiments will be described.

[3-1] Data Assignment of Memory Cell

First, data assignment to be applied to the memory system 1 according tothe third embodiment will be described with reference to FIG. 15. FIG.15 shows threshold distribution of memory cell transistors MT, assigneddata, and voltages used in write and read operations, and dataassignment is different from that shown in FIG. 10 described in thesecond embodiment.

As shown in FIG. 15, in the present embodiment, data is assigned to thememory cell transistors MT included in each threshold distribution asfollows:

“ER” level: “111” (“lower bit/middle bit/upper bit”) data,

“A” level: “101” data,

“B” level: “100” data,

“C” level: “110” data,

“D” level: “010” data,

“E” level: “011” data,

“F” level: “001” data, and

“G” level: “000” data.

In the read operation, the lower page data is determined by the readresult using a voltage DR. The middle page data is determined by theread results using voltages AR, CR, and FR. The upper page data isdetermined by the read results using voltages BR, ER, and GR. That is,in the data assignment in the present embodiment, the lower page data,the middle page data, and the upper page data are determined byexecuting the read operation once, three times, and three timesrespectively, similar to the data assignment in the second embodiment.The data assignment used in the present embodiment is also referred toas a “1-3-3 code”, similar to the second embodiment.

[3-2] Write Operation of Memory System 1

Next, a write operation of the memory system 1 according to the thirdembodiment will be described. In the write operation in the thirdembodiment, the details of a first write operation and a second writeoperation are different from the write operation described in the firstembodiment.

First, the details of the first write operation in the third embodimentwill be described with reference to FIG. 16. FIG. 16 shows 2-page dataused in the first write operation and threshold distributions of thememory cell transistors MT before and after the first write operation isexecuted.

As shown in FIG. 16, the arrangement of the corresponding thresholddistribution after the write operation is different between the firstwrite operation in the third embodiment and the first write operationdescribed in the second embodiment with reference to FIG. 13.

Specifically, a semiconductor memory device 10 sets the memory celltransistors MT to which “11” (“lower bit/middle bit”) data is written asa write inhibit state, and executes the write operations using voltagesM1V, M2V, and M3V as verify voltages to the memory cell transistors MTto which “10” data, “01” data, and “00” data are written respectively.The voltage M1V is lower than the voltage BR. The voltage M2V is higherthan the voltage M1V and lower than the voltage ER. The voltage M3V ishigher than voltage M2V and lower than voltage GR. Thus, the thresholdvoltages of the memory cell transistors MT to which the “11” data iswritten are distributed to the “ER” level, the threshold voltages of thememory cell transistors MT to which the “10” data is written aredistributed to the “M1” level, the threshold voltages of the memory celltransistors MT to which the “01” data is written are distributed to the“M2” level, and the threshold voltages of the memory cell transistors MTto which the “00” data is written are distributed to the “M3” level.

The threshold voltage of the memory cell transistor MT included in the“M1” level is equal to or higher than the voltage M1V and lower than thevoltage BR, the threshold voltage of the memory cell transistor MTincluded in the “M2” level is equal to or higher than the voltage M2Vand lower than the voltage ER, and the threshold voltage of the memorycell transistor MT included in the “M1” level is equal to or higher thanthe voltage M3V and lower than the voltage GR. The verify voltages M1V,M2V, and M3V are set so that the threshold voltages of the memory celltransistors MT having passed the verify do not exceed the voltages BR,ER, and GR respectively. The verify voltages M1V and M2V may be set sothat the interval between the threshold distribution at the “M1” leveland the threshold distribution at the “M2” level is wider than theintervals between other threshold distributions.

Next, the details of the second write operation in the third embodimentwill be described with reference to FIG. 17. FIG. 17 shows 3-page dataused in the second write operation and threshold distributions of thememory cell transistors MT before and after the second write operationis executed.

As shown in FIG. 17, the correspondence relationship between thethreshold distributions before and after the second write operation isdifferent between the second write operation in the third embodiment andthe second write operation described in the second embodiment withreference to FIG. 14.

Specifically, in the second write operation, a sequencer 14 sets thememory cell transistors MT to which “111” (“lower bit/middle bit/upperbit”) data is written as a write inhibit state, and executes the writeoperations using the voltages AV, BV, CV, DV, EV, FV, and GV as verifyvoltages to the memory cell transistors MT to which “101” data, “100”data, “110” data, “010” data, “011” data, “001” data, and “000” data arerespectively written. Thus, the threshold distribution at the “C” levelis formed from the threshold distribution at the “ER” level, thethreshold distributions at the “A” level and “B” level are formed fromthe threshold distribution at the “M1” level, the thresholddistributions at the “D” level and the “E” level are formed from thethreshold distribution at the “M2” level, and the thresholddistributions at the “F” level and “G” level are formed from thethreshold distribution at the “M3” level.

[3-3] Effects of Third Embodiment

The memory system 1 according to the third embodiment described aboveexecutes a two-stage write operation to which the 1-3-3 code differentfrom that in the second embodiment is applied.

In such a case, the memory system 1 can suppress the influences of theinitial fall of the threshold voltage and the parasitic capacitancebetween the adjacent memory cells which occur after data is written,similar to the second embodiment. Accordingly, the memory system 1according to the third embodiment can further suppress the expansion ofthe threshold distribution in the writing operation as compared with thefirst embodiment, and improve the reliability of the written data.

In the memory system 1 according to the third embodiment, the 1-3-3 codeshown in FIG. 15 is applied as the assignment of data to be written tothe memory cell transistor MT. In this manner, with the 1-3-3 code shownin FIG. 15, it is possible to obtain effects similar to the case ofapplying the 1-3-3 code shown in FIG. 10. The memory system 1 accordingto the third embodiment can omit data input for one page in the secondwrite operation, and accelerate the write operation similar to thesecond embodiment. Furthermore, since the memory system 1 according tothe third embodiment can discard the lower page data used by thecontroller 20 in the first write operation after transmitting the lowerpage data to the semiconductor memory device 10, the memory system 1 cancontrol the storage capacity of a RAM 22 and a buffer memory 24 similarto the second embodiment, and control the circuit area of the controller20.

In the third embodiment, the sequencer 14 controls the interval betweenthe threshold distribution at the “M1” level and the thresholddistribution at the “M2” level in the four threshold distributionsformed in the first write operation so as to be wider than the intervalsbetween other threshold distributions, similar to the second embodiment.That is, the memory system 1 according to the third embodiment cansuppress the number of read error bits in the IDL similar to the secondembodiment, and can suppress the deterioration of the reliability of thedata to be written in the second write operation.

[4] Fourth Embodiment

Next, a memory system 1 according to a fourth embodiment will bedescribed. In the memory system 1 according to the fourth embodiment,data assignment different from that in the memory system 1 described inthe second embodiment is applied, and a controller 20 executes aconversion process of write data. Hereinafter, differences between thememory system 1 according to the fourth embodiment and that according tothe first to third embodiments will be described.

[4-1] Data Assignment of Memory Cell

First, data assignment to be applied to the memory system 1 according tothe fourth embodiment will be described with reference to FIG. 18. FIG.18 shows threshold distribution of memory cell transistors MT, assigneddata, and voltages used in write and read operations, and dataassignment is different from that shown in FIG. 10 described in thesecond embodiment.

As shown in FIG. 18, in the present embodiment, data is assigned to thememory cell transistors MT included in each threshold distribution asfollows:

“ER” level: “111” (“lower bit/middle bit/upper bit”) data,

“A” level: “011” data,

“B” level: “001” data,

“C” level: “000” data,

“D” level: “010” data,

“E” level: “110” data,

“F” level: “100” data, and

“G” level: “101” data.

In the read voltage, the lower page data is determined by the readresults using voltages AR and ER. The middle page data is determined bythe read results using voltages BR, DR, and FR. The upper page data isdetermined by the read results using voltages CR and GR. That is, in thedata assignment according to the present embodiment, the lower pagedata, the middle page data, and the upper page data are determined byexecuting read operations twice, three times, and twice respectively.Hereinafter, such data assignment is referred to as a “2-3-2 code”.

[4-2] Write Operation of Memory System 1

Next, a write operation of the memory system 1 according to the fourthembodiment will be described with reference to FIG. 19. FIG. 19 shows aflowchart of the write operation of the memory system 1. Steps S50 toS64 shown in FIG. 19 correspond to steps S30 to S44 respectivelydescribed in the second embodiment with reference to FIG. 11. In thepresent embodiment, differences from the second embodiment are that thecontroller 20 executes the conversion process of the write data beforetransmitting first and second command sets, and that the converted writedata is written by first and second write operations.

As shown in FIG. 19, the controller 20 executes a first data conversionprocess (step ST1) before transmitting the first command set to asemiconductor memory device 10 (for example, before step S50). Thecontroller 20 further executes a second data conversion process (stepST2) before transmitting the second command set to the semiconductormemory device 10 (for example, before step S56).

Here, the first and second data conversion processes will be describedwith reference to FIG. 20. FIG. 20 shows data assignment to be appliedto write data received from a host apparatus 30 and various types ofwrite data generated from the write data.

As shown in FIG. 20, the controller 20 applies the 2-3-2 code to thewrite data received from the host apparatus 30. Then, the controller 20generates first write data to be used in the first write operation bythe first data conversion process from the write data to which the 2-3-2code is applied, and generates second write data to be used in thesecond write operation by the second data conversion process.

In the first write data constituted by two pages, “11” (first lowerbit/first upper bit after data conversion) data is assigned to the datacorresponding to the “ER” level and the “A” level, “10” data is assignedto the data corresponding to the “B” level and the “C” level, “00” datais assigned to the data corresponding to the “D” level and the “E”level, and “01” data is assigned to the data corresponding to the “F”level and the “G” level. Hereinafter, among the 2-page data, the lowerpage data after the data conversion is referred to as a first lower pagedata ML1, and the upper page data after the data conversion is referredto as a first upper page data MU1.

In the second write data constituted by two pages, “11” (second lowerbit/second upper bit after data conversion) data is assigned to the datacorresponding to the “ER” level and the “G” level, “10” data is assignedto the data corresponding to the “A” level and the “F” level, “00” datais assigned to the data corresponding to the “B” level and the “E”level, and “01” data is assigned to the data corresponding to the “C”level and the “D” level. Hereinafter, among the 2-page data, the lowerpage data after the data conversion is referred to as a second lowerpage data ML2, and the upper page data after the data conversion isreferred to as a second upper page data MU2.

As described above, in the present embodiment, the data assignment basedon the 2-3-2 code is different from the data assignment based on thefirst and second write data.

Next, the details of the first write operation in the fourth embodimentwill be described with reference to FIG. 21. FIG. 21 shows 2-page dataused in the first write operation and threshold distributions of thememory cell transistors MT before and after the first write operation isexecuted.

As shown in FIG. 21, the threshold voltages of the memory celltransistors MT before the execution of the first write operation aredistributed to the “ER” level, which is an erase state. In the firstwrite operation, the semiconductor memory device 10 executes the writeoperation for 2-page data based on the first lower page data ML1 and thefirst upper page data MU1 input from the controller 20, and forms fourthreshold distributions from the threshold distribution at the “ER”level.

Specifically, the semiconductor memory device 10 sets the memory celltransistor MT to which “11” (“first lower bit/first upper bit after dataconversion”) data is written as a write inhibit state, and executes thewrite operation using voltages M1V, M2V, and M3V as verify voltages tothe memory cell transistors MT to which “10” data, “00” data, and “01”data are written respectively. The voltage M1V is lower than the voltageCR. The voltage M2V is higher than the voltage M1V and lower than thevoltage ER. The voltage M3V is higher than voltage M2V and lower thanvoltage GR. Thus, the threshold voltages of the memory cell transistorsMT to which the “11” data is written are distributed to the “ER” level,the threshold voltages of the memory cell transistors MT to which the“10” data is written are distributed to the “M1” level, the thresholdvoltages of the memory cell transistors MT to which the “00” data iswritten are distributed to the “M2” level, and the threshold voltages ofthe memory cell transistors MT to which the “01” data is written aredistributed to the “M3” level.

The threshold voltage of the memory cell transistor MT included in the“M1” level is equal to or higher than the voltage M1V and lower than thevoltage CR, the threshold voltage of the memory cell transistor MTincluded in the “M2” level is equal to or higher than the voltage M2Vand lower than the voltage ER, and the threshold voltage of the memorycell transistor MT included in the “M1” level is equal to or higher thanthe voltage M3V and lower than the voltage GR. The verify voltages M1V,M2V, and M3V are set so that the threshold voltages of the memory celltransistors MT having passed the verify do not exceed the voltages CR,ER, and GR respectively. The verify voltages M1V and M2V may be set sothat the interval between the threshold distribution at the “M1” leveland the threshold distribution at the “M2” level is wider than theintervals between other threshold distributions.

Next, the details of the second write operation in the fourth embodimentwill be described with reference to FIG. 22. FIG. 22 shows 3-page dataused in the second write operation and threshold distributions of thememory cell transistors MT before and after the second write operationis executed.

As shown in FIG. 22, the threshold voltages of the memory celltransistors MT before the execution of the second write operation aredistributed to the “ER” level, the “M1” level, the “M2” level, and “M3”level. In the second write operation, the semiconductor memory device 10executes the write operation for 3-page data based on the second lowerpage data ML2 and the second upper page data MU2 input from thecontroller 20 and the first lower page data ML1 read from a memory cellarray 11, and forms eight threshold distributions from the thresholddistributions at the “ER” level, the “M1” level, the “M2” level, and the“M3” level.

Specifically, a sequencer 14 first executes an internal data load (IDL).In the IDL in the present embodiment, a read operation using a voltageM2R is executed. The voltage M2R is a voltage set between the thresholddistribution at the “M1” level and the threshold distribution at the“M2” level. Therefore, since the lower page data at the “ER” level andthe “M1” level is “1” and the first lower page data ML1 at the “M2”level and the “M3” level is “0”, a sense amplifier module 17 candetermine whether the lower page data is “1” or “0” by determiningwhether or not the threshold voltage of the memory cell transistor MT islower than the voltage M2R. In this manner, the semiconductor memorydevice 10 restores “1” data and “0” data (first lower page data ML1)written by the first write operation to a latch circuit in a senseamplifier unit SAU.

Next, the sequencer 14 sets the memory cell transistors MT to which“111” (“first lower bit/second lower bit/second upper bit after dataconversion”) data is written as a write inhibit state, and executes thewrite operations using the voltages AV, BV, CV, DV, EV, FV, and GV asverify voltages to the memory cell transistors MT to which “110” data,“100” data, “101” data, “001” data, “000” data, “010” data, and “011”data are written respectively. Thus, the threshold distribution at the“A” level is formed from the threshold distribution at the “ER” level,the threshold distributions at the “B” level and “C” level are formedfrom the threshold distribution at the “M1” level, the thresholddistributions at the “D” level and the “E” level are formed from thethreshold distribution at the “M2” level, and the thresholddistributions at the “F” level and “G” level are formed from thethreshold distribution at the “M3” level.

In the example shown in FIG. 19, it has been described that the writeoperation (for example, after steps S51, S54, S57, S60, and S63) isexecuted, and then the next command set and data are input after aready/busy signal RBn becomes the “H” level, but the operation order isnot limited thereto. For example, by having an extra number of latchcircuits (for example, latch circuits XDL) described with reference toFIG. 2 for write cache usage, it is possible to input the next commandset and data during the write operation.

The command sequence in the write operation described above is similarto the command sequence described in the second embodiment withreference to FIG. 12 in that the content of the write data transmittedfrom the controller 20 is changed. Specifically, the first command setused in the present embodiment is similar in that the lower page dataand the middle page data in the first command set shown in FIG. 12 areconverted to the first lower page data ML1 and the first upper page dataMU1 respectively. The second command set used in the present embodimentis similar in that the middle page data and the upper page data in thesecond command set shown in FIG. 12 are respectively converted to thesecond lower page data ML2 and the second upper page data MU2.

[4-3] Effects of Fourth Embodiment

The memory system 1 according to the fourth embodiment described abovefurther executes the data conversion process to the write data receivedfrom the host apparatus 30 and to which data is assigned, and executesthe first and second write operations similar to those in the second andthird embodiments using the converted data. As a result, in the memorysystem 1 according to the fourth embodiment, the data to be read in theread operation is similar to the write data before the data conversion.Thus, the memory system 1 according to the fourth embodiment canselectively execute the data assignment suitable for the read operationand the data assignment suitable for the write operation.

Specifically, in the memory system 1 according to the fourth embodiment,the data assignment shown in FIG. 20 is respectively applied to thefirst and second write data after the data conversion used in first andsecond write operations. Since the first write data is evenly assignedto the eight threshold distributions, it is possible to write the datato the memory cell transistor MT to a high threshold voltage in thefirst write operation. That is, in the second write operation in thefourth embodiment, it is possible to suppress the variation amount ofthe threshold voltage, similar to the second and third embodiments, andto suppress the influences of the initial fall of the threshold voltageand the parasitic capacitance between the adjacent memory cells whichoccur after data is written.

In the memory system 1 according to the fourth embodiment, the 2-3-2code shown in FIG. 18 is applied as the data assignment to the writedata before the data conversion process. In the 2-3-2 code, the numberof read times for determining each page data is averaged, and bydetermining the data by reading at the voltages BR, DR, and FR at whicherror bits are relatively hard to generate in the read operation for themiddle page which needs to be read three times, an increase in thenumber of error bits due to the increase in the number of read times issuppressed. Thus, the memory system 1 can disperse the number of errorbits generated in the read operations for the lower page, the middlepage, and the upper page in the 3-page data read operation, and enhancethe possibility of succeeding in the error correction by the ECC circuit25.

The memory system 1 according to the fourth embodiment described abovecan suppress the expansion of the threshold distribution in the writeoperation similar to the second and third embodiments, and suppress thenumber of error bits in the read operation. Accordingly, the memorysystem 1 according to the fourth embodiment can further improve thereliability of the written data and the reliability of the readoperation, as compared with the first embodiment.

As described above, in the fourth embodiment, since the data assignmentduring the read operation is different from that during the writeoperation, the data transmitted from the controller 20 to thesemiconductor memory device 10 during the write operation is differentfrom the data transmitted from the semiconductor memory device 10 to thecontroller 20 during the read operation.

The second write data described above may be assigned as shown in FIG.23. Specifically, as shown in FIG. 23, “11” (second lower bit/secondupper bit after data conversion) data is assigned to the datacorresponding to the “ER” level and the “F” level, “10” data is assignedto the data corresponding to the “A” level and the “G” level, “01” datais assigned to the data corresponding to the “B” level and the “D”level, and “00” data is assigned to the data corresponding to the “C”level and the “E” level. In such a case, it is possible to execute thewrite operation described in the fourth embodiment.

The memory system 1 according to the fourth embodiment executes thesecond write operation using the IDL, similar to the second and thirdembodiments. Accordingly, the memory system 1 according to the fourthembodiment can shorten the time of data input in the second writeoperation, and can accelerate the write operation similar to the secondand third embodiments.

The memory system 1 according to the fourth embodiment controls theinterval between the threshold distribution at the “M1” level and thethreshold distribution at the “M2” level in the four thresholddistributions formed in the first write operation so as to be wider thanthe intervals between other threshold distributions, similar to thesecond and third embodiments. Accordingly, the memory system 1 accordingto the present embodiment can suppress the number of read error bitsgenerated in the IDL, similar to the second and third embodiments, andsuppress the deterioration of the reliability of the data to be writtenin the second write operation.

[5] Fifth Embodiment

Next, a memory system 1 according to a fifth embodiment will bedescribed. In the fifth embodiment, a data conversion process differentfrom that in the memory system 1 described in the fourth embodiment isexecuted, and the IDL of 2-page data is executed in a second writeoperation using a soft bit. Hereinafter, differences between the memorysystem 1 according to the fifth embodiment and that according to thefirst to fourth embodiments will be described.

[5-1] Write Operation of Memory System 1

Hereinafter, a write operation of the memory system 1 according to thefifth embodiment will be described. The write operation in the fifthembodiment is different from the write operation described in the fourthembodiment in details of a write data conversion process, a first writeoperation, and a second write operation.

First, the data conversion process in the fifth embodiment will bedescribed with reference to FIG. 24. FIG. 24 shows data assignment to beapplied to write data received from a host apparatus 30 and varioustypes of write data converted from the write data.

As shown in FIG. 24, the data conversion process in the fifth embodimentis different in the assignment of second write data from the dataconversion process described in the fourth embodiment with reference toFIG. 20. Specifically, in the second write data, “11” (second lowerbit/second upper bit after data conversion) data is assigned to the datacorresponding to an “ER” level and a “D” level, “10” data is assigned tothe data corresponding to an “A” level and an “E” level, “00” data isassigned to the data corresponding to a “B” level and an “F” level, and“01” data is assigned to the data corresponding to a “C” level and a “G”level.

Next, the first write operation in the fifth embodiment will bedescribed with reference to FIG. 25. FIG. 25 shows 2-page data used inthe first write operation and threshold distributions of the memory celltransistors MT before and after the first write operation is executed.

As shown in FIG. 25, in the first write operation in the fifthembodiment, the write operation for 2-page data is executed based on afirst command set including first lower page data ML1 and first upperpage data MU1. In the first write operation in the fifth embodiment, theinterval between the threshold distributions is different from that inthe first write operation described in the fourth embodiment withreference to FIG. 21. Specifically, in the first write operation in thefifth embodiment, verify voltages M1V, M2V, and M3V are set so that thethreshold distributions at, for example, the “ER” level, the “M1” level,the “M2” level, and the “M3” level are evenly arranged as much aspossible. Since the other operations are similar to the operationsdescribed in the fourth embodiment with reference to FIG. 21, thedescription thereof is omitted.

Next, the second write operation in the fifth embodiment will bedescribed with reference to FIG. 26. FIG. 26 shows 4-page data used inthe second write operation and threshold distributions of the memorycell transistors MT before and after the second write operation isexecuted.

As shown in FIG. 26, the threshold voltages of the memory celltransistors MT before the execution of the second write operation aredistributed to the “ER” level, the “M1” level, the “M2” level, and the“M3” level. In the second write operation, a semiconductor memory device10 executes write operation for 3-page data based on second upper pagedata MU2 and second lower page data ML2 input from a controller 20, andthe first lower page data ML1 and the first upper page data MU1corrected using a soft bit, and forms eight threshold distributions fromthe threshold distributions at the “ER” level, the “M1” level, the “M2”level, and the “M3” level.

Specifically, the sequencer 14 first executes an internal data load(IDL). In the IDL in the fifth embodiment, the read operation usingvoltages M1R, M2R, and M3R is executed. The voltage M1R is a voltage setbetween the threshold distribution at the “ER” level and the thresholddistribution at the “M1” level, the voltage M2R is a voltage set betweenthe threshold distribution at the “M1” level and the thresholddistribution at the “M2” level, and the voltage M1R is a voltage setbetween the threshold distribution at the “ER” level and the thresholddistribution at the “M1” level.

A sense amplifier part SA determines whether the threshold voltage ofthe memory cell transistor MT is included in which thresholddistribution at the “ER” level, the “M1” level, the “M2” level, or the“M3” level by the read operation using the voltages M1R, M2R, and M3R,and transfers the determined 2-bit data to a latch circuit in a senseamplifier unit SAU. Then, an operation unit OP executes a datacorrection process using the read 2-bit data, the second lower page dataML2 received from the controller 20, and a soft bit. The soft bitindicates data read using a read voltage shifted to the plus side or theminus side as compared with the ordinary read voltage.

Here, an example of the data correction process using the second lowerpage data ML2 and the soft bit will be described with reference to FIG.27. FIG. 27 shows a table for explaining the data correction process andthreshold distribution for explaining a soft bit read executed in thedata correction process.

In the second write operation, it can be regarded that the second lowerpage data ML2 received from the controller 20 does not include errorbits. On the other hand, the 2-page data read from a memory cell array11 during the IDL probably includes error bits. Taking these points intoconsideration, eight kinds of data shown in FIG. 27 can be listed asdata retained in the latch circuit of each sense amplifier unit SAU.

Since data “111” (first lower bit/first upper bit/second lower bit afterdata conversion) data, “100” data, “001” data, and “010” data shown inFIG. 27 match the combination of the data shown in FIG. 26, it isconsidered that the data is read correctly. On the other hand, since“110” data, “101” data, “000” data, and “010” data shown in FIG. 27 donot match the combination of the data shown in FIG. 26, the data isregarded as error data.

Among the error data, the “101” data and the “011” data are consideredto correctly be either “001” data or “111” data because the second lowerbit data is regarded as correct data. Similarly, the “110” data and the“000” data among the error data are considered to correctly be either“100” data or “010” data. Based on these points, the sequencer 14executes the first and second soft bit read, and the error data iscorrected by the operation unit OP.

In the first soft bit read, the “101” data and the “011” data arecorrected. Specifically, the sequencer 14 executes the read operationusing a read voltage S1R to the memory cell transistor MT in which thedata is determined as the “101” data and the “011” data, and theoperation unit OP corrects the error data. The voltage S1R is higherthan the voltage M1R and lower than the voltage M2R, and is set to, forexample, a voltage included in the “M1” level. By this read operation,the sense amplifier part SA determines whether the threshold voltage ofthe memory cell transistor MT is included in which thresholddistribution at the “ER” level or the “M2” level. Based on the readresult, the operation unit OP corrects the data in the memory celltransistor MT determined as the “101” data and the “011” data to the“111” data when the threshold voltage thereof is equal to or lower thanthe voltage S1R, and corrects the data to the “001” data when thethreshold voltage exceeds the voltage S1R.

In the second soft bit read, the “110” data and the “000” data arecorrected. Specifically, the sequencer 14 executes the read operationusing a read voltage S2R to the memory cell transistor MT in which thedata is determined as the “110” data and the “000” data, and theoperation unit OP corrects the error data. The voltage S2R is higherthan the voltage M2R and lower than the voltage M3R, and is set to, forexample, a voltage included in the “M2” level. By this read operation,the sense amplifier part SA determines whether the threshold voltage ofthe memory cell transistor MT is included in which thresholddistribution at the “M1” level or the “M3” level. Based on the readresult, the operation unit OP corrects the data in the memory celltransistor MT determined as the “100” data and the “010” data to the“100” data when the threshold voltage thereof is equal to or lower thanthe voltage S2R, and corrects the data to the “010” data when thethreshold voltage exceeds the voltage S2R.

The details of the read operation using the soft bit described above aredisclosed in, for example, U.S. patent application Ser. No. 12/504,966entitled “Semiconductor memory device and semiconductor storage system”filed on Jul. 17, 2009. It is also disclosed in U.S. patent applicationSer. No. 14/688,442 entitled “Semiconductor memory device” filed on Apr.16, 2015. These patent applications are incorporated by reference hereinin their entirety.

As described above, the “11” (first lower bit/first upper bit after dataconversion) data, the “10” data, the “00” data, and the “01” datawritten by the first write operation are restored to the latch circuitin the sense amplifier unit SAU by the IDL using the second lower pagedata ML2 and the soft bit.

The sequencer 14 sets the memory cell transistors MT to which “111”(“first lower bit/first upper bit/second upper bit after dataconversion”) data is written as a write inhibit state, and executes thewrite operations using the voltages AV, BV, CV, DV, EV, FV, and GV asthe verify voltages to the memory cell transistors MT to which “110”data, “100” data, “101” data, “001” data, “000” data, “010” data, and“011” data are written respectively. Thus, the threshold distribution atthe “A” level is formed from the threshold distribution at the “ER”level, the threshold distributions at the “B” level and “C” level areformed from the threshold distribution at the “M1” level, the thresholddistributions at the “D” level and the “E” level are formed from thethreshold distribution at the “M2” level, and the thresholddistributions at the “F” level and “G” level are formed from thethreshold distribution at the “M3” level.

In the first and second write operations described above, the waveformof the voltage applied to a selected word line WL is, for example, asshown in FIG. 28. FIG. 28 shows an example of an input/output signal I/Oand a voltage applied to a selected word line WL.

As shown in FIG. 28, the write operation in the fifth embodiment isdifferent from the write operation described in the first embodimentwith reference to FIG. 9 in the waveform of a selected word line WL_selduring the IDL period in the second write operation.

Specifically, in the second write operation, first, a row decoder 16sequentially applies the read voltages M1R, M2R, and M3R to the selectedword line WL_sel. Subsequently, the row decoder 16 sequentially appliesthe read voltages S1R and S2R to the selected word line WL_sel. Theperiods during the voltages M1R, M2R, M3R, S1R, and S2R that are appliedto the selected word line WL_sel correspond to the IDL.

The order and the timing of applying each read voltage during the IDLperiod are not limited thereto. For example, the voltages S1R and S2Rcorresponding to the soft bit read may not be applied consecutively.Alternatively, the read voltages may be applied in the order of, forexample, the voltages M1R, S1R, M2R, S2R, and M3R during the IDL period.The order of executing the normal read operation and the soft bit readduring the IDL period is not particularly limited. Since the otheroperations are similar to the operations described in the firstembodiment with reference to FIG. 9, the description thereof is omitted.

[5-2] Effects of Fifth Embodiment

The memory system 1 according to the fifth embodiment described aboveexecutes the IDL of 2-page data using 1-page data received from thecontroller 20 and the soft bit in the second write operation describedin the fourth embodiment.

Thus, the memory system 1 according to the fifth embodiment can correcterrors of 2-page data read by the IDL. Accordingly, the memory system 1according to the fifth embodiment can improve the reliability of thewrite data in the second write operation using the IDL of 2-page data,and improve the reliability of the written data.

The memory system 1 according to the fifth embodiment can suppress theexpansion of the threshold distribution in the write operation similarto the fourth embodiment, and can suppress the number of error bits inthe read operation. Accordingly, the memory system 1 according to thefifth embodiment can further improve the reliability of the written dataand the reliability of the read operation, as compared with the firstembodiment.

The second write data described above may be assigned as shown in FIG.29. Specifically, as shown in FIG. 29, “11” (second lower bit/secondupper bit after data conversion) data is assigned to the datacorresponding to the “ER” level and the “D” level, “10” data is assignedto the data corresponding to the “A” level and the “E” level, “01” datais assigned to the data corresponding to the “B” level and the “F”level, and “00” data is assigned to the data corresponding to the “C”level and the “F” level. In such a case, it is possible to execute thewrite operation described in the fifth embodiment.

In the memory system 1 according to the fifth embodiment, the data to beread in the read operation is similar to the write data before the dataconversion, similar to the fourth embodiment. In the present embodimentas well, the data transmitted from the controller 20 to thesemiconductor memory device 10 during the first write operation isdifferent from the data transmitted from the semiconductor memory device10 to the controller 20 during the read operation, similar to the fourthembodiment.

[6] Sixth Embodiment

Next, a memory system 1 according to a sixth embodiment will bedescribed. The memory system 1 according to the sixth embodimentexecutes the second write operation in the fourth embodiment based on3-page data received from a controller 20. Hereinafter, differencesbetween the memory system 1 according to the sixth embodiment and thoseaccording to the first to fifth embodiments will be described.

[6-1] Write Operation of Memory System 1

Hereinafter, a write operation of the memory system 1 according to thesixth embodiment will be described with reference to FIG. 30. FIG. 30shows a flowchart of the write operation of the memory system 1. StepsS70 to S84 shown in FIG. 30 correspond to steps S50 to S64 respectivelydescribed in the fourth embodiment with reference to FIG. 19. The writeoperation in the sixth embodiment is different from the write operationdescribed in the fourth embodiment in details of a write data conversionprocess, and a command sequence and a second write operation.

As shown in FIG. 30, in the write operation in the sixth embodiment, asecond data conversion process before a controller 20 transmits a secondcommand set to a semiconductor memory device 10 is omitted, as comparedwith the operation described in the fourth embodiment with reference toFIG. 19. Here, a data conversion process in the sixth embodiment will bedescribed with reference to FIG. 31. FIG. 31 shows data assignment to beapplied to write data received from a host apparatus 30, and varioustypes of write data converted from the write data.

As shown in FIG. 31, in the sixth embodiment, the controller 20 appliesa 2-3-2 code to the write data received from the host apparatus 30,similar to the fourth embodiment. Then, the controller 20 executes afirst data conversion process similar to that in the fourth embodimentto the write data to which the 2-3-2 code is applied. That is, thememory system 1 according to the sixth embodiment uses first write datasimilar to that in FIG. 20 in a first write operation.

Next, a command sequence of the write operation in the sixth embodimentwill be described with reference to FIG. 32. FIG. 32 shows the commandsequence of the operation corresponding to FIG. 30.

As shown in FIG. 32, a first command set in the sixth embodiment issimilar to that the lower page data, and the middle page data in thefirst command set shown in FIG. 12 in the second embodiment arerespectively converted to first lower page data ML1 and first upper pagedata MU1. On the other hand, a second command set in the sixthembodiment is similar in that the set of commands “80h” to “xxh” in thesecond command set shown in FIG. 12 is repeated twice. In the secondcommand set in the sixth embodiment, for example, the write data to betransmitted before the two commands of “xxh” and the command “10h” islower page data, middle page data, and upper page data.

Next, the second write operation in the sixth embodiment will bedescribed with reference to FIG. 33. FIG. 33 shows 3-page data used inthe second write operation and threshold distributions of memory celltransistors MT before and after the second write operation is executed.

As shown in FIG. 33, the threshold voltages of the memory celltransistors MT before the execution of the second write operation aredistributed to an “ER” level, an “M1” level, an “M2” level, and an “M3”level. In the second write operation, the semiconductor memory device 10executes the write operation for 3-page data based on the lower pagedata, the middle page data, and upper page data MU2 input from thecontroller 20, and forms eight threshold distributions from thethreshold distributions at the “ER” level, the “M1” level, the “M2”level, and the “M3” level.

Specifically, the semiconductor memory device 10 sets the memory celltransistors MT to which “111” (“lower bit/middle bit/upper bit”) data iswritten as a write inhibit state, and executes the write operationsusing voltages AV, BV, CV, DV, EV, FV, and GV as verify voltages to thememory cell transistors MT to which “110” data, “100” data, “101” data,“001” data, “000” data, “010” data, and “011” data are respectivelywritten. Thus, the threshold distribution at the “A” level is formedfrom the threshold distribution at the “ER” level, the thresholddistributions at the “B” level and “C” level are formed from thethreshold distribution at the “M1” level, the threshold distributions atthe “D” level and the “E” level are formed from the thresholddistribution at the “M2” level, and the threshold distributions at the“F” level and “G” level are formed from the threshold distribution atthe “M3” level.

In the example shown in FIG. 30, it has been described that the writeoperation (for example, after steps S71, S74, S77, S80, and S83) isexecuted, and then the next command set and data are input after aready/busy signal RBn becomes the “H” level, but the operation order isnot limited to thereto. For example, by having an extra number of latchcircuits (for example, latch circuits XDL) described with reference toFIG. 2 for write cache usage, it is possible to input the next commandset and data during the write operation.

[6-2] Effects of Sixth Embodiment

The memory system 1 according to the sixth embodiment described aboveexecutes the second write operation using 3-page data to which the dataconversion process is not executed in the write operation described inthe fourth embodiment.

Thus, in the memory system 1 according to the sixth embodiment, it isunnecessary to adjust the interval between the threshold distributionsin the first write operation in consideration of executing the IDL inthe second write operation in the second embodiment.

The memory system 1 according to the sixth embodiment can suppress theinfluences of the initial fall of the threshold voltage and theparasitic capacitance between the adjacent memory cells that occur afterdata is written, similar to the fourth embodiment. Accordingly, thememory system 1 according to the sixth embodiment can further suppressthe expansion of the threshold distribution in the writing operation ascompared with the first embodiment, and can improve the reliability ofthe written data.

In the sixth embodiment, it has been described that the data conversionprocess is not executed to the write data used in the second writeoperation, but the operation is not limited to this. For example, 3-pagedata to which the data conversion process is executed may be used in thesecond write operation. In this case, the conversion process is executedto the 3-page data, for example, before the controller 20 transmits thesecond command set to the semiconductor memory device 10.

In the memory system 1 according to the sixth embodiment, the data to beread in the read operation is similar to the write data before the dataconversion, which is similar to the fourth embodiment. That is, in thesixth embodiment, the data transmitted from the controller 20 to thesemiconductor memory device 10 during the first write operation isdifferent from the data transmitted from the semiconductor memory device10 to the controller 20 during the read operation.

[7] Seventh Embodiment

Next, a memory system 1 according to a seventh embodiment will bedescribed. The memory system 1 according to the seventh embodimentrelates to the writing order when the write operation described in thefirst to sixth embodiments is applied to a semiconductor memory device10 having a structure in which memory cells are three-dimensionallystacked. Hereinafter, differences between the memory system 1 accordingto the seventh embodiment and those according to the first to sixthembodiments will be described.

[7-1] Configuration of Memory Cell Array 11

First, a configuration example of a memory cell array 11 in the seventhembodiment will be described with reference to FIG. 34. FIG. 34 is acircuit diagram of the memory cell array 11 and shows a detailed circuitconfiguration of one block BLK in the memory cell array 11. The circuitconfiguration of the memory cell array 11 in the seventh embodiment isdifferent, in that a plurality of string units SU is provided in eachblock BLK, from the circuit configuration of the memory cell array 11described using FIG. 2 in the first embodiment.

Specifically, the block BLK includes, for example, string units SU0 toSU3 as shown in FIG. 34. Each of the string units SU includes m numberof NAND strings NS. Gates of select transistors ST1 in the string unitsSU0 to SU3 are commonly connected to select gate lines SGD0 to SGD3respectively. Drains of the select transistors ST1 on the same column inthe same block are commonly connected to a corresponding bit line BL.That is, the drains of the select transistors ST1 on the same column arecommonly connected between the blocks BLK. Control gates of memorytransistors MT0 to MT7 in the same block are commonly connected to wordlines WL0 to WL7 respectively. Gates of select transistors ST2 in thesame block are commonly connected to a select gate line SGS. Sources ofthe select transistors ST2 in the same block are commonly connected to asource line SL. That is, the sources of the select transistors ST2 arecommonly connected between the blocks BLK. Since the other circuitconfigurations are similar to the circuit configurations described inthe first embodiment with reference to FIG. 2, the description thereofis omitted.

Next, an example of the cross-sectional configuration of the memory cellarray 11 will be described with reference to FIG. 35. FIG. 35 shows across-sectional view of the memory cell array 11, and X, Y, and Z axescrossing each other. In FIG. 35, the interlayer insulating film isomitted. As shown in FIG. 35, the semiconductor memory device 10includes a P-type well region 60, conductors 61 to 66, a plurality ofsemiconductor pillars MH, and a plurality of contact plugs LI.

The P-type well region 60 is formed in the surface of the semiconductorsubstrate. The conductors 61 to 63 are laminated in order above theP-type well region 60. The conductors 61 to 63 function as the selectgate line SGS, the word line WL, and the select gate line SGDrespectively.

For example, the number of layers of the conductors 62 corresponds tothe number of word lines WL. In the example shown in FIG. 35, the selectgate lines SGS and SGD are each constituted by four layers ofconductors. The select gate lines SGS and SGD may be constituted by aplurality of conductors as described above, or may be constituted by onelayer of conductor. The conductors 61 to 63 are provided in a plateshape extending in the X direction and the Y direction.

The semiconductor pillars MH are formed so as to extend from the uppersurface of the conductor 63 to the upper surface of the P-type wellregion 60. The semiconductor pillars MH are provided so as to passthrough the conductors 61 to 63 along the Z direction. On the side faceof each semiconductor pillar MH, a block insulating film 67, aninsulating film (charge storage layer) 68, and a tunnel oxide film 69are formed in order. In the semiconductor pillar MH, a semiconductormaterial 70 including a conductive material is buried at the inner sideof the tunnel oxide film 69. The inner side of the tunnel oxide film 69of the semiconductor pillar MH may be constituted by a plurality ofmaterials.

The conductor 64 corresponding to the bit line BL is formed above theconductor 63 and the semiconductor pillar MH. The bit line BL isconnected to the corresponding semiconductor pillar MH. A contact plugcontaining a conductive material may be formed between the bit line BLand the corresponding semiconductor pillar MH.

The conductors 65 and 66 corresponding to the source line SL and a wellline CPWELL respectively are formed between the conductors 63 and 64.The conductor 65 is connected via the contact plug LI to an n⁺ typeimpurity diffusion region 71 formed in the surface of the well region60. The conductor 66 is connected via the contact plug LI to a p⁺ typeimpurity diffusion region 72 formed in the surface of the well region60. The contact plug LI is provided in a plate shape extending in the Xdirection and the Z direction.

In the above configuration, one semiconductor pillar MH corresponds toone NAND string NS. Specifically, the intersections between the selectgate line SGD and the semiconductor pillar MH, and between the selectgate line SGS and the semiconductor pillar MH correspond to the selecttransistors ST1 and ST2, respectively. Similarly, the intersectionbetween the word line WL and the semiconductor pillar MH corresponds tothe memory cell transistor MT.

Furthermore, a plurality of above configurations is arranged in the Xdirection. For example, one string unit SU is constituted by a group ofNAND strings NS arranged in the X direction. When a plurality of stringunits SU is provided in the same block BLK, the conductors 63corresponding to the select gate line SGD are separated between thestring units SU.

The lowermost conductor 61 and the tunnel oxide film 69 are provided inthe vicinity of the n⁺ type impurity diffusion region 71. Accordingly,when the select transistor ST2 is turned ON, a current path is formedbetween the NAND string NS and the n⁺ type impurity diffusion region 71.

The memory cell array 11 may have other configurations.

A memory cell array formation may be disclosed in U.S. patentapplication Ser. No. 14/532,030 filed on Mar. 23, 2009, the entirecontents of which are incorporated by reference herein.

Furthermore a memory cell array formation may be disclosed in U.S.patent application Ser. No. 12/679,991 filed on Mar. 25, 2010, theentire contents of which are incorporated by reference herein.

A memory cell array formation may be disclosed in U.S. patentapplication Ser. No. 14/406,524 filed on Mar. 18, 2009, the entirecontents of which are incorporated by reference herein.

Additionally, a memory cell array formation may be disclosed in U.S.patent application Ser. No. 12/407,403 filed on Mar. 19, 2009, theentire contents of which are incorporated by reference herein.

[7-2] Write Operation of Memory System 1

Next, a write operation of the memory system 1 will be described withreference to FIG. 36. FIG. 36 is a flowchart for explaining a writingorder in the seventh embodiment. In the following description, variablesi and j are used for ease of explanation. The variables i and j are keptby a counter of a controller 20 and incremented by, for example, controlfrom the controller 20.

As shown in FIG. 36, first, the semiconductor memory device 10 executesa first write operation in which a word line WLi (i=0) is selected andthe string units SU0 to SU3 are sequentially selected (step S90). Whenthe first write operation in step S90 is ended, the variable i isincremented, and the variable j is reset (j=0) (step S91). Then, thesemiconductor memory device 10 executes the first write operation inwhich the word line WLi is selected and a string unit SUj is selected(step S92). Specifically, the semiconductor memory device 10 executesthe first write operation in which the word line WL1 is selected and thestring unit SU0 is selected.

Next, the semiconductor memory device 10 executes a second writeoperation in which a word line WL(i−1) is selected and a string unit SUjis selected (step S93). Specifically, the semiconductor memory device 10executes the second write operation in which the word line WL0 isselected and the string unit SU0 is selected.

When j=3 is not satisfied once the second write operation in step S93 isended (NO in step S94), the variable j is incremented (step S95), andstep S82 and subsequent operations are repeated. On the other hand, whenj=3 is satisfied (YES in step S94), the value of the variable i iscontinuously checked (step S96).

When i=7 is not satisfied (NO in step S96), the operation returns tostep S91. After the variable i is incremented and the variable j isreset, step S92 and subsequent operations are repeated. On the otherhand, when i=7 is satisfied (YES in step S96), the semiconductor memorydevice 10 executes the second write operation in which a word line WLi(i=7) is selected and the string units SU0 to SU3 are sequentiallyselected (step S97).

FIG. 37 shows the writing order in each write operation described above.FIG. 37 shows combinations of word lines WL and string units SU in acertain block BLK. FIG. 37 also shows a frame (white background)corresponding to the first write operation and a frame (hatchedbackground) corresponding to the second write operation in eachcombination, and the number representing the order for executing theoperation in each frame. The solid arrow shown in FIG. 37 indicates thatthe operation at the starting point of the arrow is the first writeoperation, and the dotted arrow indicates that the operation at thestarting point of the arrow is the second write operation.

As shown in FIG. 37, the semiconductor memory device 10 executes thefirst write operation to the string units SU in which the word line WL0is selected, and then alternately executes the first write operation inwhich the word line WL1 is selected, and the second write operation inwhich the word line WL0 is selected. This operation is executed suchthat the string units SU0 to SU3 are sequentially selected. The memorysystem 1 executes the second write operation in which the word line WL0and the string unit SU3 are selected, and then alternately executes thefirst write operation in which the word line WL2 is selected, and thesecond write operation in which the word line WL1 is selected. The sameapplies hereinafter.

[7-3] Effects of Seventh Embodiment

The memory system 1 according to the seventh embodiment described abovecan suppress the storage capacity of a RAM 22. Hereinafter, effects ofthe seventh embodiment will be described in detail.

When the two-stage write operation described in the first to sixthembodiments is applied to a memory system in which each block BLK has aplurality of string units SU, the order shown in FIG. 38 is consideredas the writing order of the first write operation and the second writeoperation.

In a comparative example of the seventh embodiment shown in FIG. 38,after the first write operation in which the word line WL0 and eachstring unit SU are selected, the first write operation is executed inwhich the word line WL1 and each string unit SU are selected. After thefirst write operation is executed in which the word line WL1 and eachstring unit SU are selected, the second write operation is executed inwhich the word line WL0 and each string unit SU are selected. That is,in the memory system in the comparative example, the order for executingthe first write operation and the second write operation is managed inthe unit of word line WL.

On the other hand, in the present embodiment, after the first writeoperation in which the word line WL0 and each string unit SU areselected, the first write operation is executed in which the word lineWL1 and the string unit SU0 are selected as shown in FIG. 37. Then,after the first write operation is executed in which the word line WL1and the string unit SU0 are selected, the second write operation isexecuted in which the word line WL0 and the string unit SU0 areselected. As described above, in the present embodiment, the memorysystem 1 sequentially executes the second write operation from thestring unit SU in which the first write operation with respect to theadjacent word line WL is ended.

In the write operation of the memory system 1 according to the presentembodiment described above, since the period until the second writeoperation is executed is shorter than that in the comparative example,the period during which the RAM 22 retains the write data is shortened.Accordingly, the memory system 1 according to the present embodiment cansuppress the consumption amount of the RAM 22 and the storage capacityof the RAM 22.

The writing order with which the period until the second write operationis executed is shortened may be the order shown in FIG. 39. In avariation of the seventh embodiment shown in FIG. 39, after the firstwrite operation in which the word line WL0 and the string unit SU0 areselected, the first write operation is executed in which the word lineWL1 and the string unit SU0 are selected. Then, after the first writeoperation is executed in which the word line WL1 and the string unit SU0are selected, the second write operation is executed in which the wordline WL0 and the string unit SU0 are selected. The writing order iscontinuously used until the second write operation is executed in whichthe word line WL0 and the string unit SU3 are selected. The subsequentwriting order is similar to FIG. 37 described in the present embodiment.The memory system 1 can similarly suppress the storage capacity of theRAM 22 when such a writing order is applied.

In the above description, it has been described that each block BLKincludes the four string units SU, but the number of string units is notlimited thereto. For example, the number of the string units SU may betwo, three, or five or more. The determination values of the variable jin step S94 and of the variable i in step S96 used in the description inthe flowchart of FIG. 36 are merely examples, and the determinationvalue is not limited thereto. These determination values are set basedon, for example, the number of the string units SU corresponding to oneblock BLK or the number of the word lines WL.

[8] Eighth Embodiment

Next, a memory system 1 according to an eighth embodiment will bedescribed. The eighth embodiment relates to a write method applicable tothe write operation described in the first to seventh embodiments, inwhich the reference of a verify pass at each level is set to a lowerverify voltage used in a write operation by a quick pass write (QPW)method. Hereinafter, differences between the memory system 1 accordingto the eighth embodiment and that according to the first to seventhembodiments will be described.

[8-1] Data Assignment of Memory Cell

First, data assignment to be applied to the memory system 1 according tothe eighth embodiment will be described with reference to FIG. 40. FIG.40 shows threshold distribution of memory cell transistors MT, assigneddata, and voltages used in the write operation.

As shown in FIG. 40, in the eighth embodiment, it will be described thatan MLC method for storing 2-bit data in one memory cell transistor MT isapplied. When data is written by the MLC method, threshold distributionof the memory cell transistors MT is divided into four. These fourthreshold distributions are referred to as an “ER” level, an “A” level,a “B” level, and a “C” level in order from a lower threshold voltage.

The “ER” level corresponds to an erase state of the memory celltransistor MT. The “A” level, the “B” level, and the “C” levelcorrespond to write states of the memory cell transistor MT, and areformed by write operations using verify voltages AV, BV, and CVrespectively. In the write operation by the QPW method, a verify voltageAVL lower than the verify voltage AV at the “A” level and a verifyvoltage BVL lower than the verify voltage BV at the “B” level are used.The relation between these voltage values is as follows:AVL<AV<BVL<BV<CV<DV<EV<FV<GV.

In the eighth embodiment, “10” (lower bit/upper bit) data, “11” data,“01” data, and “00” data are respectively written to the memory celltransistors MT at the “ER” level, the “A” level, the “B” level, and the“C” level. The data assignment in the present embodiment is called a“1-2 code” because the lower bit and the upper bit are determined byrespectively executing the read operation once and twice.

[8-2] Write Operation of Memory System 1

Next, the write operation of the memory system 1 will be described. Thememory system 1 according to the eighth embodiment uses the writeoperation by the quick pass write (QPW) method. In the write operationby the QPW method, two verify voltages are used in the verify operationfor the memory cell transistor MT in which data at each level iswritten.

Specifically, as shown in, for example, FIG. 40, the two verify voltagesAV and AVL are used in the verify operation at the “A” level, and thetwo verify voltages BV and BVL are used in the verify operation at the“B” level. When it is detected by repeating a program loop that thethreshold voltage of the memory cell transistor MT is between the lowerverify voltage and the normal verify voltage, the next program operationis executed while an intermediate voltage is being applied to a bit lineBL corresponding to the memory cell transistor MT. When the programoperation is executed while the intermediate voltage is being applied tothe bit line BL in this manner, the potential difference between thegate and the channel of the memory cell transistor MT becomes small, andthus the variation of the threshold voltage is suppressed as comparedwith the case in which the voltage of the bit line BL is set to theground voltage during the program operation.

As described above, in the program operation by the QPW method, theprogram operation in which the variation range of the threshold voltageis large and the program operation in which the variation range of thethreshold voltage is small are switched in accordance with approachingthe target threshold voltage. Accordingly, in the write operation by theQPW method, it is possible to narrow the width of the thresholddistribution after the write operation while the increase in the writetime is suppressed.

The details of the writing operation by the QPW method are disclosed in,for example, U.S. patent application Ser. No. 14/263,948 entitled“Nonvolatile semiconductor memory device” filed on Apr. 28, 2014. It isalso described in U.S. patent application Ser. No. 12/563,296 entitled“Nonvolatile semiconductor memory device” filed on Sep. 21, 2009. Thesepatent applications are incorporated by reference herein in theirentirety.

In the write operation of the memory system 1 according to the eighthembodiment, the lower verify voltage (for example, voltage AVL) in theQPW method is used as a reference of the verify pass. Hereinafter, thedetails of the write operation in the eighth embodiment will bedescribed with reference to FIGS. 41 and 42. FIG. 41 shows a flowchartof the write operation in the eighth embodiment, and FIG. 42 showsexamples of voltages applied to a selected word line WL_sel and a bitline BL during the write operation. In the example shown in FIG. 42, thememory cell transistor MT passes the verify at the “A” level, the “B”level, and the “C” level after the program pulse is respectively appliedtwice, four times, and six times. In the following description, a bitline BL to be written is referred to as a bit line BL_prog, awrite-inhibited bit line BL is referred to as a bit line BL_inh, and abit line BL to which the QPW method is applied is referred to as a bitline BL_qpw.

As shown in FIG. 41, a sequencer 14 first executes a program operation(step S100). Specifically, as shown in FIG. 42, a sense amplifier module17 applies a voltage Vss to the bit line BL_prog, and applies a voltageVbl to the bit line BL_inh. The voltage Vss is the ground voltage, andthe voltage Vbl is the power supply voltage supplied from the senseamplifier module 17. When the voltage Vbl is applied to the bit lineBL_inh, a corresponding select transistor ST1 is cut off, and thechannel of a corresponding NAND string NS is in a floating state. Then,a row decoder 16 applies a program voltage Vpgm to the selected wordline WL_sel. Then, in the memory cell transistor MT connected to the bitline BL_prog, the threshold voltage varies due to the potentialdifference between the control gate and the channel. Thus, in the memorycell transistor MT connected to the bit line BL_inh, the variation ofthe threshold voltage is suppressed due to channel boost or the like.

Next, the sequencer 14 executes the verify operation at the “A” level(step S101). Specifically, the row decoder 16 sequentially applies theverify voltages AVL and AV to the selected word line WL_sel, and thesense amplifier module 17 determines whether the threshold voltage ofthe memory cell transistor MT is equal to or higher than the voltagesAVL and AV. This verify result is counted while the subsequentoperations are being executed. Subsequently, the sequencer 14 incrementsthe program voltage Vpgm by ΔVpgm (step S102) and executes the programoperation again (step S103).

Next, the sequencer 14 checks the count result of the verify operationin step S101, and determines whether or not the number of memory cellshaving the threshold voltage lower than the verify voltage AVL in thememory cell transistors MT to which the data at the “A” level is writtenis equal to or less than a specified value (step S104). This specifiedvalue can be set to a given numerical value.

When the number of memory cells lower than the verify voltage AVL islarger than the specified value (NO in step S103), the operation returnsto step S101 to execute the verify operation at the “A” level and theprogram operation in which the program voltage Vpgm is incremented. Inthe second and subsequent program loops, the write operation to whichthe QPW method is applied is executed to the memory cell transistor MThaving passed the verify operation using the lower verify voltage (forexample, the verify voltage AVL). Specifically, while the senseamplifier module 17 is applying an intermediate voltage Vqpw to the bitline BL_qpw, the row decoder 16 applies the program voltage Vpgm to theselected word line WL_sel. The voltage Vqpw is higher than the voltageVss and lower than the voltage Vbl. Then, the threshold voltage of thememory cell transistor MT connected to the bit line BL_qpw varies morefinely than the case in which the voltage Vss is applied to the bit lineBL.

When the number of memory cells lower than the verify voltage AVL isequal to or less than the specified value (YES in step S103), thesequencer 14 determines that the memory cell transistor MT has passedthe verify at the “A” level. Then, the sequencer 14 executes the verifyoperation at “B” level (step S105). Specifically, the row decoder 16sequentially applies the verify voltages BVL and BV to the selected wordline WL_sel, and the sense amplifier module 17 determines whether thethreshold voltage of the memory cell transistor MT is equal to or higherthan the voltages BVL and BV. This verify result is counted while thesubsequent operations are being executed. Subsequently, the sequencer 14increments the program voltage Vpgm by ΔVpgm (step S106) and executesthe program operation again (step S107).

Next, the sequencer 14 checks the count result of the verify operationin step S105, and determines whether the number of memory cells havingthe threshold voltage lower than the verify voltage BVL in the memorycell transistors MT to which the data corresponding to the “B” level iswritten is less than a specified value (step S108). This specified valuecan be set to an arbitrary numerical value.

When the number of memory cells lower than the verify voltage BVL islarger than the specified value (NO in step S108), the operation returnsto step S105 to execute the verify operation at the “B” level and theprogram operation in which the program voltage Vpgm is incremented.

When the number of memory cells lower than the verify voltage BVL isequal to or less than the specified value (YES in step S108), thesequencer 14 determines that the memory cell transistor MT has passedthe verify at the “B” level. Then, the sequencer 14 executes the verifyoperation at the CV level (step S109). Specifically, the row decoder 16applies the verify voltage CV to the selected word line WL_sel, and thesense amplifier module 17 determines whether the threshold voltage ofthe memory cell transistor MT is equal to or higher than the voltage CV.This verify result is counted while the subsequent operations are beingexecuted. Subsequently, the sequencer 14 increments the program voltageVpgm by ΔVpgm (step S110) and executes the program operation again (stepS111).

Next, the sequencer 14 checks the count result of the verify operationin step S109, and determines whether or not the number of memory cellshaving the threshold voltage lower than the verify voltage CV in thememory cell transistors MT to which the data corresponding to the “C”level is written is equal to or less than the specified value (stepS112).

When the number of memory cells lower than the verify voltage CV islarger than the specified value (NO in step S112), the operation returnsto step S109 to execute the verify operation at the “C” level, and theprogram operation in which the program voltage Vpgm is incremented.

When the number of memory cells lower than the CV is equal to or lessthan the specified value (YES in step S112), the sequencer 14 determinesthat the memory cell transistor MT has passed the verify at the “C”level, and ends the write operation.

[8-3] Effects of Eighth Embodiment

The memory system 1 according to the eighth embodiment described abovecan improve reliability of written data. Hereinafter, effects of theeighth embodiment will be described in detail with reference to FIG. 43.FIG. 43 shows an example of a change in the threshold distribution ofthe memory cell transistors MT corresponding to the waveforms shown inFIG. 42. The threshold distribution having the white background shown inFIG. 43 corresponds to the threshold distribution of the memory celltransistors MT to which data has been written, and the thresholddistribution having the hatched background corresponds to the thresholddistribution of the writing target memory cell transistors MT.Furthermore, (1) in FIG. 43 shows the threshold distribution before thewrite operation, and (2) to (8) show the threshold distributions afterthe program voltage is respectively applied in the first to seventhprogram loops.

As shown in FIG. 43, the threshold distribution of the memory celltransistors MT rises each time the program voltage is applied. Theverify operation after the program voltage is applied is counted whilethe next program voltage is being applied. Thus, in the verifyoperation, when the threshold distribution is likely to exceed thespecified verify voltage by applying the next program voltage, thesequencer 14 determines that the corresponding memory cell transistor MThas passed the verify. With respect to the verify pass in the data ateach level, the case in which the number of memory cell transistors MTequal to or lower than the verify voltage (for example, voltages AV andBV) in the memory cell transistors MT to which the data at thecorresponding level is written is less than the specified value, isdetermined as the verify pass.

However, when the number of memory cell transistors MT equal to or lowerthan the verify voltage is counted as the criterion for determination ofthe verify operation, the number of target memory cell transistors MT islarge, and the counting accuracy can deteriorate. In addition, when thenumber of memory cell transistors MT to be counted is large, currentconsumption may significantly increase.

Therefore, in the memory system 1 according to the eighth embodiment,the lower verify voltage (for example, the voltages AVL and BVL) used inthe write operation by the QPW method is used as a criterion fordetermination of the verify operation. For example, as shown in (3) ofFIG. 43, since the number of the memory cell transistors MT equal to orlower than the voltage AVL is less than the number of the memory celltransistors MT equal to or lower than the voltage AV, the number of thememory cells to be counted at the time of determination of the verifypass can be reduced. This point applies to the verify operations atother levels.

Thus, the memory system 1 according to the eighth embodiment can reducethe number of the memory cell transistor MT to be counted as comparedwith the case of applying the normal verify voltage, and improve thecounting accuracy. Accordingly, the memory system 1 according to theeighth embodiment can improve the accuracy of the verify operation andthe reliability of the written data. Furthermore, the memory system 1according to the eighth embodiment can reduce the number of memory celltransistors MT to be counted, and suppress the power consumption.

In the eighth embodiment, it has been described that two verify voltagesare used in the write operation by the QPW method, but the number ofvoltages is not limited thereto. For example, if one verify voltage isused in the write operation by the QPW method, by changing the lengthand timing of the sense time, it is possible to obtain a verify resultsimilar to the case of executing the verify operation with a low verifyvoltage.

In the eighth embodiment, it has been described that the verifyoperation for the subsequent level is started in accordance with passingthe verify operation at a lower level, but the verify operation is notlimited thereto. For example, the number of program loop times fromwhich the verify operation at the corresponding level is executed may bedetermined in advance by a parameter based on the write characteristicof the memory cell transistor MT.

Furthermore, in the eighth embodiment, it has been described that theverify operation at any one of the “A” level, the “B” level, or the “C”level is executed in each program loop, but the verify operation is notlimited thereto. For example, as shown in FIG. 44, a plurality of verifyoperations may be executed in one program loop. FIG. 44 shows examplesof the voltages applied to the selected word line WL_sel during thewrite operation. In the examples shown in FIG. 44, the verify operationsat the “A” level and the “B” level are continuously executed in thethird program loop, and the verify operations at the “B” level and the“C” level is continuously executed in the fifth program loop. In thememory system 1 described above, an appropriate verify operation isexecuted in accordance with the threshold voltage of the memory celltransistor MT that rises as the program loop progresses. In other words,the memory system 1 can improve the verify accuracy in the case ofcontinuously executing the verify operations at a plurality of levels,by counting the number of memory cell transistors MT equal to or lowerthan the lower verify voltage used in the QPW method.

[9] Ninth Embodiment

Next, a memory system 1 according to a ninth embodiment will bedescribed. The ninth embodiment relates to a write method applicable tothe write operation described in the first to eighth embodiments, inwhich a program voltage is applied a predetermined number of times afterpassing the verify at a lower level. Hereinafter, differences betweenthe memory system 1 according to the ninth embodiment and that accordingto the first to eighth embodiments will be described.

[9-1] Write Operation of Memory System 1

Hereinafter, the details of a write operation in the ninth embodimentwill be described with reference to FIGS. 45 and 46. FIG. 45 shows aflowchart of the write operation in the ninth embodiment, and FIG. 46shows examples of voltages applied to a selected word line WL_sel and abit line BL during the write operation. In the example shown in FIG. 46,a memory cell transistor MT passes the verify at an “A” level after thefifth program pulse is applied.

As shown in FIG. 45, a sequencer 14 first executes a program operation(step S120). Next, the sequencer 14 executes a verify operation at the“A” level (step S121). Specifically, as shown in FIG. 46, a row decoder16 applies a verify voltage AV to the selected word line WL_sel, and asense amplifier module 17 determines whether or not the thresholdvoltage of the memory cell transistor MT is equal to or higher than thevoltage AV. This verify result is counted while the subsequentoperations are being executed. Subsequently, the sequencer 14 incrementsa program voltage Vpgm by ΔVpgm (step S122) and executes the programoperation again (step S123).

Next, the sequencer 14 checks the count result of the verify operationin step S121, and determines whether or not the number of memory cellshaving the threshold voltage lower than the verify voltage AV in thememory cell transistors MT to which the data corresponding to the “A”level is written is equal to or less than a specified value (step S124).This specified value can be set to a given numerical value.

When the number of memory cells lower than the verify voltage AV islarger than the specified value (NO in step S124), the operation returnsto step S121 to execute the verify operation at the “A” level, and theprogram operation in which the program voltage Vpgm is incremented.

When the number of memory cells lower than the voltage AV is equal to orless than the specified value (YES in step S124), the sequencer 14determines that the memory cell transistor MT has passed the verify atthe “A” level, and the verify operation is omitted in the subsequentprogram loops.

Then, the sequencer 14 increments the program voltage Vpgm by ΔVpgm(step S125) and executes the program operation (step S126). Next, thesequencer 14 determines whether or not the program voltage have beenapplied a specified number of times after the memory cell transistor MThas passed the verify at the “A” level (step S127). This specifiednumber of times is set to the number at which the threshold voltage ofthe memory cell transistor MT to which the data at the “C” level iswritten in the memory cell transistors MT having the threshold voltagelower than the verify voltage at the “A” level is expected to exceed thevoltage CV.

When the program voltage has not been applied the specified number oftimes after the memory cell transistor MT has passed the verify at the“A” level (NO in step S127), the operation returns to step S125, and theprogram operation in which the program voltage Vpgm is incremented againis executed. When the program voltage has been applied the specifiednumber of times after the memory cell transistor MT has passed theverify at the “A” level (YES in step S127), the sequencer 14 determinesthat the threshold voltage of the writing target memory cell transistorMT reaches a desired level, and the write operation is ended.

FIG. 47 shows an example of the change in the threshold distribution ofthe memory cell transistors MT corresponding to FIG. 46 in the writeoperation described above. The horizontal axis in FIG. 47 indicates thenumber of program loops, and the vertical axis indicates the thresholdvoltage Vth. In FIG. 47, the threshold distribution of the memory celltransistors MT in an erase state (“ER” level) is divided into fourgroups based on the initial value of the threshold voltages. These fourgroups are referred to as groups Gr1 to Gr4 in the descending order ofthe initial values of the threshold voltages. In FIG. 47, therelationship between the number of program loops for each group and thethreshold voltage is plotted with black circles. In the example shown inFIG. 47, it is assumed that the memory cell transistor MT has passed theverify at the “A” level after executing the fifth program loop.

In the ninth embodiment as shown in FIG. 47, it is assumed that theincrease width of the threshold voltage by one program loop is fixed.Then, the sequencer 14 determines whether writing of the data at the “B”level and the “C” level in each memory cell transistor MT is ended basedon the number of application times of the program voltage after thememory cell transistor MT has passed the verify at the “A” level.Specifically, in the example shown in FIG. 47, the threshold voltage ofthe memory cell transistor MT exceeds the verify voltage AV at the “A”level, then exceeds the verify voltage BV at the “B” level by executingthe program loop three times, and then exceeds the verify voltage CV atthe “C” level by further executing the program loop three times.

The sequencer 14 further determines that the threshold voltage of thememory cell transistor MT in the group Gr4, which does not exceed theverify voltage at the “A” level at the time of passing the verify at the“A” level, exceeds the verify voltage at the “A” level by applying theprogram voltage one more time. That is, as the criterion fordetermination of the number of memory cells used in step S124 in theninth embodiment, the number of the memory cell transistors MT expectedto have the lowest threshold voltage in the threshold distribution inthe erased state is set to the number at which the threshold voltage isexpected to exceed the verify voltage AV at the “A” level by executingthe program operation one more time.

In this case, in the write operation at the “B” level, the sequencer 14determines that the threshold voltage of the memory cell transistor MTexceeds the verify voltage AV at the “A” level and then exceeds theverify voltage BV at the “B” level after the program loop is executedthree times, and that the threshold voltage of the memory celltransistor MT in the group Gr4 exceeds the verify voltage BV at the “B”level after the program loop is executed four times (three+one times).Similarly, in the write operation at the “C” level, the sequencer 14determines that the threshold voltage of the memory cell transistor MTexceeds the verify voltage AV at the “A” level and then exceeds theverify voltage CV at the “C” level after the program loop is executedsix times (three+three times), and that the threshold voltage of thememory cell transistor MT in the group Gr4 exceeds the verify voltage CVat the “C” level after the program loop is executed seven times(three+three+one times).

[9-2] Effects of Ninth Embodiment

As described above, the memory system 1 according to the ninthembodiment omits the verify operation after the number of the memorycell transistors MT that have passed the verify of the data at the “A”level is less than the predetermined number. Then, the memory system 1executes, for the memory cell transistor MT having passed the verify atthe “A” level, the program loop in which the verify operation is omittedby the number of times set for each data to be written.

Furthermore, in the memory system 1 according to the ninth embodiment,when data at a desired level is written, the program operation isexecuted for the memory cell transistor MT which has not passed theverify at the “A” level at the time of omitting the verify at the “A”level one more time than the memory cell transistor MT that has passedthe verify at the “A” level. In other words, the memory system 1 omitsone verify operation by reducing the number of the memory celltransistors MT for determining whether the writing of the data at the“A” level has ended.

Thus, the memory system 1 according to the ninth embodiment can shortenthe time for one verify operation, and can accelerate the writeoperation.

In the ninth embodiment, it has been described that the write operationsat the “B” level and the “C” level are executed using the verify pass atthe “A” level as a criterion, but the criterion for the verify pass isnot limited thereto. For example, similar effects can be obtained byexecuting the verify operations at the “A” level and “B” level andsetting the criterion for the verify pass at the “B” level as thesetting described in the ninth embodiment.

[10] Tenth Embodiment

Next, a memory system 1 according to a tenth embodiment will bedescribed. In the tenth embodiment, the QPW method is applied to aprogram operation which is likely to exceed a verify voltage at adesired level in the write operation described in the ninth embodiment.Hereinafter, differences between the memory system 1 according to thetenth embodiment and that according to the first to ninth embodimentswill be described.

[10-1] Write Operation of Memory System 1

Hereinafter, a write operation of the memory system 1 according to thetenth embodiment will be described below with reference to FIGS. 48 and49. FIG. 48 shows examples of voltages applied to a selected word lineWL_sel and a bit line BL during the write operation, and FIG. 49 showsan example of a change in threshold distribution of memory celltransistors MT corresponding to FIG. 48. The waveforms of the bit linesBL_prog in groups Gr1 and Gr4 shown in FIG. 48 correspond to those inthe case of writing of data at a “C” level to the memory celltransistors MT in the groups Gr1 and Gr4 shown in FIG. 49.

In the examples shown in FIGS. 48 and 49, the memory cell transistor MTin the group Gr1 has passed the verify at an “A” level by the thirdprogram loop. Then, the memory cell transistor MT in the group Gr4reaches the threshold voltage to exceed the “A” level by the one moreprogram loop after the fifth program loop. As a result, in the sixth andsubsequent program loops, the verify operation is omitted.

As shown in FIGS. 48 and 49, in the tenth embodiment, the writeoperation by the QPW method is applied in, for example, the writeoperation at the “C” level corresponding to the data in the highestthreshold distribution in the MLC method.

Specifically, as shown in FIGS. 48 and 49, the threshold voltage of thememory cell transistor MT in, for example, the group Gr1 rises to thevicinity of the voltage CV by the ninth program loop. Thus, a sequencer14 executes the write operation by the QPW method in the tenth programloop. That is, as shown in FIG. 48, the write operation is executedwhile an intermediate voltage Vqpw is being applied to the bit lineBL_prog in the group Gr1. Thus, as shown in FIG. 49, the rising amountof the threshold voltage is suppressed, and the writing is ended in astate of slightly exceeding the voltage CV. In the subsequent programloops, the bit line BL in the group Gr1 is set as a write inhibit state,and a voltage Vbl is applied when a program voltage is applied. Thesequencer 14 similarly applies the write operation by the QPW method tothe memory cell transistors MT in the groups Gr2 to Gr4 in the programloop expected to reach the vicinity of the target threshold voltage inthe subsequent program loops.

[10-2] Effects of Tenth Embodiment

The memory system 1 according to the tenth embodiment described abovecan improve reliability of write data. Hereinafter, the details of theeffects of the tenth embodiment will be described.

The data retention of the memory cell transistor MT tends to becomeworse as the threshold voltage deviates from the neutral thresholdvalue, that is, as the threshold voltage rises. Therefore, there is acase in which the difference between the verify voltage BV and theverify voltage CV (voltage CV-voltage BV) needs to be larger than thedifference between the verify voltage AV and the verify voltage BV(voltage BV-voltage AV).

In addition, the threshold voltage of the memory cell transistor MThardly rises as the threshold voltage rises. Thus, the rising width ofthe threshold voltage is not expected to be fixed after, for example,exceeding the verify voltage BV at the “B” level in the write operationby the MLC method. For this reason, in order to write data at the “C”level corresponding to the highest threshold distribution, the programloop can be executed four times which is one more than the exampledescribed in the ninth embodiment with reference to FIG. 47.

However, when the number of program loops is simply increased in thismanner, overwriting in which the specified verify voltage is largelyexceeded can occur. In the overwrite state, the threshold distributionin the erased state extends when the data in the corresponding page iserased for the next time.

Therefore, the memory system 1 according to the tenth embodimentexecutes the write operation by the QPW method in a program loopexpected to exceed a desired level in the write operation described inthe ninth embodiment Thus, the memory system 1 according to the tenthembodiment can suppress overwriting in the write operation at the “C”level and further narrow the threshold distribution, and thereby improvethe reliability of the write data.

In the above description, it has been described that the write operationby the QPW method is applied only to the writing at the “C” level, butthe applied level is not limited thereto. For example, as shown in FIG.50, such a write operation may be applied to the write operation at the“B” level. FIG. 50 shows an example of a change in the thresholddistribution of the memory cell transistors MT corresponding to FIG. 49.In FIG. 50, the change in the threshold voltage of the memory celltransistor MT, to which data at the “B” level is written, is indicatedby solid lines and black circles, and the change in the thresholdvoltage of the memory cell transistor MT, to which data at the “C” levelis written, is indicated by broken lines and white circles. As shown inFIG. 50, the memory system 1 can narrow the threshold distribution atthe “B” level by applying the write operation by the QPW method to theprogram loop expected to reach the “B” level. Accordingly, the memorysystem 1 can improve the reliability of write data.

The memory system 1 according to the tenth embodiment omits one verifyoperation by reducing the number of the memory cell transistors MT fordetermining whether the writing the data at the “A” level has ended,similar to the ninth embodiment. Accordingly, the memory system 1according to the tenth embodiment can accelerate the writing operation.

[11] Eleventh Embodiment

Next, an eleventh embodiment will be described. In a known method, inorder to distinguish whether a program for a word line WLi is before asecond write operation or after the second write operation, writing isexecuted to the flag memory cell (flag cell) at the time of the secondwrite operation. The memory system may proceed that a reading sequenceis appropriately controlled based on the information recorded in theflag cell at the time of data reading. The memory system 1 according tothe eleventh embodiment may use no flag cell. In the eleventhembodiment, since data is written after a second write operation at an“ER” level or higher of the threshold voltage of a memory cell in thenormal write operation, it is determined that the second write operationhas been executed if the data is written at the “ER” level or higher ofthe threshold voltage of the memory cell, and that the second writeoperation has not been executed if the data is not written at the “ER”level or higher of the threshold voltage of the memory cell.

[11-1] Data Assignment of Memory Cell

First, data assignment to be applied to a memory system 1 according tothe eleventh embodiment will be described with reference to FIG. 51.FIG. 51 shows threshold distribution of memory cell transistors MT,assigned data, and voltages used in write and read operations, and dataassignment is different from that shown in FIG. 10 described in thesecond embodiment.

As shown in FIG. 51, in the eleventh embodiment, data is assigned to thememory cell transistors MT included in each threshold distribution asfollows:

“ER” level: “111” (“lower bit/middle bit/upper bit”) data,

“A” level: “101” data,

“B” level: “100” data,

“C” level: “110” data,

“D” level: “010” data,

“E” level: “011” data,

“F” level: “001” data, and

“G” level: “000” data.

At the read voltage, the lower page data is determined by the readresult using a voltage DR. The middle page data is determined by theread results using voltages AR, CR, and FR. The upper page data isdetermined by the read result using voltages BR, ER, and GR. That is, inthe data assignment according to the present embodiment, the lower pagedata, the middle page data, and the upper page data are determined byrespectively executing read operations once, three times, and threetimes. Hereinafter, such data assignment is referred to as a “1-3-3code”.

[11-2] Write Operation of Memory System 1

The write operation of the memory system 1 according to the presentembodiment is similar to the operation described in the first embodimentwith reference to FIGS. 5 and 6.

Next, the details of the first write operation in the present embodimentwill be described with reference to FIG. 52. FIG. 52 shows 1-page dataused in a first write operation, and threshold distributions of thememory cell transistors MT before and after the first write operation isexecuted.

As shown in FIG. 52, the threshold voltages of the memory celltransistors MT before the execution of the first write operation aredistributed to the “ER” level which is an erase state. In the firstwrite operation, a semiconductor memory device 10 executes the writeoperation for 1-page data based on the lower page data input from acontroller 20, and forms two threshold distributions from the thresholddistribution at the “ER” level.

Specifically, the semiconductor memory device 10 sets the memory celltransistor MT to which “1” data is written as a write inhibit state, andexecutes the write operation using a voltage M1V as a verify voltage tothe memory cell transistor MT to which “0” data is written. The voltageM1V is lower than the voltage ER. The verify voltage M1V is set so thatthe threshold voltage of the memory cell transistor MT having passed theverify does not exceed the voltage ER. Thereafter, the first writeoperation is similarly executed to the adjacent memory cells.

Next, the details of the second write operation in the presentembodiment will be described with reference to FIG. 53. FIG. 53 shows3-page data used in the second write operation and thresholddistributions of the memory cell transistors MT before and after thesecond write operation is executed. As shown in FIG. 53, the thresholdvoltages of the memory cell transistors MT before the execution of thesecond write operation are distributed to the “ER” level and the “M1”level. In the second write operation, the semiconductor memory device 10executes the write operation for 3-page data based on the middle andupper page data input from the controller 20 and the lower page dataread from a memory cell array 11, and forms eight thresholddistributions from the threshold distributions at the “ER” level and the“M1” level.

Specifically, the sequencer 14 first executes an internal data load(IDL). In the IDL in the present embodiment, a read operation using avoltage M1R is executed. It can be determined whether the lower pagedata is “1” or “0” by determining whether the threshold voltage of thememory cell transistor MT is lower than the voltage M1R. In this manner,the semiconductor memory device 10 restores the “1” data and “0” data(lower page data) written by the first write operation to a latchcircuit in a sense amplifier unit SAU.

The sequencer 14 sets the memory cell transistors MT to which “111” datais written as a write inhibit state, and executes the write operationsusing the voltages AV, BV, CV, DV, EV, FV, and GV as verify voltages tothe memory cell transistors MT to which “101” data, “100” data, “110”data, “010” data, “011” data, “001” data, and “000” data arerespectively written. Thus, the threshold distribution at the “A” levelis formed from the threshold distribution at the “ER” level, thethreshold distributions at the “B” level and “C” level are formed fromthe threshold distribution at the “M1” level, the thresholddistributions at the “D” level and the “E” level are formed from thethreshold distribution at the “M2” level, and the thresholddistributions at the “F” level and “G” level are formed from thethreshold distribution at the “M3” level.

In FIG. 5, it has been described that the write operation (for example,after steps S11, S14, S17, S20, and S23) is executed, and then the nextcommand set and data are input after a ready/busy signal RBn becomes the“H” level, but the operation order is not limited thereto. For example,by having an extra number of latch circuits (for example, latch circuitsXDL) described with reference to FIG. 2 for write cache usage, it ispossible to input the next command set and data during the writeoperation.

[11-3] Read Operation of Memory System 1

Here, a specific processing procedure of page reading will be described.FIGS. 54A and 54B are flowcharts showing the processing procedure ofpage reading according to the eleventh embodiment. As shown in FIG. 54,the sequencer 14 selects a read page (step S201).

When the read page is a lower page (step S201, lower), the sequencer 14executes reading using the voltage ER (step S202). Then, the sequencer14 determines whether the number of cells equal to or higher than the“E” level is equal to or larger than a specified value (step S203). Whenthe number of cells equal to or higher than the “E” level is equal to orlarger than the prescribed value (YES in step S203), the sequencer 14executes reading using the voltage DR (step S204). Thereafter, thesequencer 14 determines data based on the result read by the voltage DR(step S205). On the other hand, when the number of cells equal to orhigher than the “E” level is less than the prescribed value (NO in stepS203), the sequencer 14 executes reading using the voltage AR (stepS206). Thereafter, the sequencer 14 determines data based on the resultread by the voltage AR (step S207).

When the read page is a middle page (step S201, Middle), the sequencer14 executes reading using the voltage AR (step S208). Thereafter, thesequencer 14 executes reading using the voltage CR (step S209). Then,the sequencer 14 further executes reading using the voltage FR (stepS210). Then, the sequencer 14 determines whether or not the number ofcells equal to or higher than the “F” level is equal to or larger than aspecified value (step S211). When the number of cells equal to or higherthan the “F” level is equal to or larger than the specified value (YESin step S211), the sequencer 14 determines data based on the resultsread by the voltages AR, CR, and FR (step S212). On the other hand, whenthe number of cells equal to or higher than the “F” level is less thanthe specified value (NO in step S211), the sequencer 14 forcibly setsall output data to “1” (step S213).

When the read page is an upper page (step S201, Upper), the sequencer 14executes reading using the voltage BR (step S214). Thereafter, thesequencer 14 executes reading using the voltage ER (step S215). Then,the sequencer 14 determines whether or not the number of cells equal toor higher than the “E” level is equal to or larger than the specifiedvalue (step S216). When the number of cells equal to or higher than the“E” level is equal to or larger than the specified value (YES in stepS216), the sequencer 14 executes reading using the voltage GR (stepS217). Thereafter, the sequencer 14 determines data based on the resultread by the voltages BR, ER, and GR (step S218). On the other hand, whenthe number of cells equal to or higher than the “E” level is less thanthe specified value (NO in step S216), the sequencer 14 forcibly setsall output data to “1” (step S219).

The memory system 1 by storing information in the flag cell in thesecond write operation, it is possible to execute an appropriate readoperation in the flash memory. Thus, the external system can read thepage data in the same processing order without distinguishing whetherthe second write operation has been executed or not. In the eleventhembodiment, it is possible to read the page data in the same processingorder without distinguishing whether the second write operation has beenexecuted or not and without additionally using a flag cell.

The determination as to whether or not the number of cells equal to orhigher than the “E” level is equal to or larger than the prescribedvalue (S203), as to whether the number of cells equal to or higher thanthe “F” level is equal to or larger than the prescribed value (S211),and as to whether the number of cells equal to or higher than the “E”level is equal to or larger than the prescribed value (S216) may beexecuted based on, for example, all memory cells of 2 k to 16 kB read atthe same time, or a part of memory cells of 2 k to 16 kB.

In this method, when there is no data to be written at the “E” level,the “F” level, and the “G” level during the second write operation inthe write data, the sequencer 14 can determine that the second writeoperation has not been executed. In order to prevent such a case, it ispreferable that a randomization circuit is provided between the inputDAT and the sense amplifier module 17 of FIG. 1 in the semiconductormemory device 10. FIGS. 55A and 55B show an example of a randomizationcircuit 4. FIG. 55A shows an example of the circuit configuration of therandomization circuit 4, and FIG. 55B is a table showing an example ofrandom numbers used for randomization corresponding to the randomizationcircuit 4 in FIG. 55A. The randomization circuit 4 randomizes the inputdata using, for example, the circuit configuration and the randomnumbers as shown in FIG. 55A and FIG. 55B. Thus, the randomizationcircuit 4 can generate the data to be written at the “E” level, the “F”level, and the “G” level when there is no input data corresponding tothe “E” level, the “F” level, and the “G” level. Thus, the semiconductormemory device 10 using this method can prevent the sequencer 14 fromdetermining that the second write operation has not been executed whenthere is no data to be written at the “E” level, “F” level, and “G”level.

FIGS. 56A and 56B show a first variation of the page reading processingprocedure according to the eleventh embodiment described with referenceto FIG. 54. As shown in FIG. 56, in the present variation, the sequencer14 first selects a read page (step S221).

When the read page is a lower page (step S221, Lower), the sequencer 14executes reading using the voltage DR (step S222). Then, the sequencer14 determines whether or not the number of cells equal to or higher thanthe “D” level is equal to or larger than the specified value (stepS223). As shown in FIG. 52, since data is written at the levels lowerthan the “E” level according to the threshold distribution of the memorycell after the first write operation, if the determination is executedwith the “D” level, the sequencer 14 can determine that the second writeoperation has been executed. In contrast to this, in the firstvariation, the sequencer 14 sets the number of specified values to belarge when it is determined whether or not the number of cells equal toor higher than the “E” level is equal to or larger than the specifiedvalue (step S203), or writes data to the memory cell having thethreshold voltage of the “D” level or lower during the first writeoperation. Thus, in the first variation, when the read operation isexecuted after the second write operation, two read operations using thevoltage ER and the voltage DR are required in the sequence of FIG. 54,but it is possible to execute the read operation by one read operationusing the voltage DR (step S222) in the sequence of FIG. 56, and toaccelerate the read time. The operations in steps S225 to S238 shown inFIG. 56 are similar to the operations in steps S205 to S218 describedwith reference to FIG. 54 respectively, and the description thereof isomitted.

FIGS. 57A and 57B show a second variation of the page reading processingprocedure according to the eleventh embodiment described with referenceto FIG. 54. The operations in steps S241 to S258 shown in FIG. 57respectively correspond to the operations in steps S201 to S218described with reference to FIG. 54. Hereinafter, differences betweenthe flowchart of the second variation shown in FIG. 57 and the flowchartshown in FIG. 54 will be described.

When the read page is a middle page, the sequencer 14 first reads thepage using the voltage FR (step S248). Thereafter, the sequencer 14determines whether the number of cells equal to or higher than the “FR”level is equal to or larger than the specified value (step S249). Atthis time, the sequencer 14 can take time to determine whether thenumber of cells equal to or higher than the “F” level is equal to orlarger than the specified number. Thus, while the read operation usingthe voltage CR is being executed (step S250), the sequencer 14determines whether the number of cells equal to or higher than the “F”level is equal to or larger than the specified number. The sequencer 14may determine whether or not the number of cells equal to or higher thanthe “F” level is equal to or larger than the specified number (stepS249) in the middle of or after executing the read operation using thevoltage CR (step S250).

When the read page is an upper page, the sequencer 14 first reads thepage using the voltage GR (step S254). Thereafter, the sequencer 14determines whether or not the number of cells equal to or higher thanthe “GR” level is equal to or larger than the specified value (stepS255). At this time, the sequencer 14 can take time to determine whetheror not the number of cells equal to or higher than the “G” level isequal to or larger than the specified number. Thus, while the readoperation using the voltage ER is being executed (step S256), thesequencer 14 determines whether or not the number of cells equal to orhigher than the “G” level is equal to or larger than the specifiednumber. The sequencer 14 may determine whether or not the number ofcells equal to or higher than the “G” level is equal to or larger thanthe specified number (step S255) in the middle of or after executing theread operation using the voltage ER (step S256).

As described above, the semiconductor memory device 10 according to thesecond variation of the eleventh embodiment concurrently executes theoperation for determining whether or not the number of cells is equal toor larger than the specified value and the read operation in the nextmemory cell, and can accelerate the reading time.

FIGS. 58A and 58B are a modified example of the processing proceduredescribed with reference to FIG. 56. While the reading is executed froma low threshold distribution in FIG. 56, the reading is executed from ahigh threshold distribution in FIG. 58. Hereinafter, differences betweenthe flowchart shown in FIG. 58 and the flowchart shown in FIG. 56 willbe described.

When the read page is a middle page, the sequencer 14 first reads thepage using the voltage FR (step S268). Thereafter, the sequencer 14determines whether or not the number of cells equal to or higher thanthe “F” level is equal to or larger than the specified value (stepS269). Thereafter, the sequencer 14 executes reading using the voltageCR (step S271) and reading using the voltage AR (step S272). At thistime, the sequencer 14 can take time to determine whether or not thenumber of cells equal to or higher than the “FR” level is equal to orlarger than the specified number. Thus, while the read operation usingthe voltage CR is being executed (step S271), the sequencer 14determines whether or not the number of cells equal to or higher thanthe “F” level is equal to or larger than the specified number. Thesequencer 14 may determine whether or not the number of cells equal toor higher than the “F” level is equal to or larger than the specifiednumber (step S269) in the middle of, or after executing the readoperation using the voltage CR (step S271). Thus, the semiconductormemory device 10 in the present example can concurrently execute theoperation for determining whether or not the number of cells is equal toor larger than the specified value and the read operation in the nextmemory cell, and can accelerate the read time.

Similarly, when the read page is an upper page, the sequencer 14 firstreads the page using the voltage GR (step S274). Thereafter, thesequencer 14 determines whether or not the number of cells equal to orhigher than the “GR” level is equal to or larger than the specifiedvalue (step S275). Thereafter, the sequencer 14 executes reading usingthe voltage ER (step S277) and reading using the voltage BR (step S278).At this time, the sequencer 14 can take time to determine whether or notthe number of cells equal to or higher than the “G” level is equal to orlarger than the specified number. Thus, while the read operation usingthe voltage ER is being executed (step S277), the sequencer 14determines whether or not the number of cells equal to or higher thanthe “G” level is equal to or larger than the specified number. Thesequencer 14 may determine whether or not the number of cells equal toor higher than the “G” level is equal to or larger than the specifiednumber (step S275) in the middle of or after executing the readoperation using the voltage ER (step S277). Thus, the semiconductormemory device 10 in the present example can concurrently execute theoperation for determining whether or not the number of cells is equal toor larger than the specified value and the read operation in the nextmemory cell, and can accelerate the read time.

FIGS. 59, 60, 61A, 61B, and 61C show an operation order of the firstwrite operation and the second write operation to the memory cellsrespectively corresponding to the word lines WL0 to WL7. FIGS. 59, 60,61A, 61B, and 61C also show a frame (white background) corresponding tothe first write operation and a frame (hatched background) correspondingto the second write operation in each combination, and show the numberrepresenting the order for executing the operation in each frame. In thepresent embodiment, the first write operation executes a 1-page dataprogram, and the second write operation executes a 2-page data program.

FIG. 59 shows an example in which the sequencer 14 executes the firstwrite operation and the second write operation to word lines provided inedge portions (for example, word lines WL0 and WL7).

FIG. 60 shows an example in which the sequencer 14 only executes thefirst write operation to word lines provided in the edge portions.

FIG. 61A shows an example in which the sequencer 14 only executes thefirst write operation to word lines provided in the edge portions, andapply the MLC method to the first write operation to the word linesprovided in the edge portions (for example, word line WL0 and WL7).

FIG. 61B shows an example in which the sequencer 14 only executes thefirst write operation to word lines provided in the edge portions, andapply the MLC method to the first write operation to the word lineprovided in one edge portion (for example, word line WL7).

FIG. 61C shows an example in which the sequencer 14 only executes thefirst write operation to word lines provided in the edge portions (forexample, word lines WL0 and WL7), and apply the MLC method to the firstwrite operation to the word line provided in another edge portion (forexample, word line WL0).

As described above, the memory system 1 according to the presentembodiment may omit the second write operation in accordance with acharacteristic of the memory cells corresponding to the word lines inthe edge portions. In this case, the memory system 1 may apply the SLCmethod or the MLC method to the memory cells corresponding to the wordlines provided in the edge portions.

[11-4] Variation of the Eleventh Embodiment

In the variation of the eleventh embodiment, one memory cell retainstwo-bit data.

FIG. 62 shows 1-page data used in the first write operation andthreshold distributions of the memory cell transistors MT before andafter the first write operation is executed in the variation of theeleventh embodiment.

FIG. 63 shows 2-page data used in the first write operation andthreshold distributions of the memory cell transistors MT before andafter the first write operation is executed in the variation of theeleventh embodiment.

FIG. 64 is flowcharts showing the processing procedure of page readingaccording to the variation of the eleventh embodiment. As shown in FIG.64, the sequencer 14 selects a read page (step S280).

When the read page is a lower page (step S280, lower), the sequencer 14executes reading using the voltage CR (step S281). Then, the sequencer14 determines whether the number of cells equal to or higher than the“C” level is equal to or larger than a specified value (step S282). Whenthe number of cells equal to or higher than the “C” level is equal to orlarger than the prescribed value (YES in step S203), the sequencer 14executes reading using the voltage BR (step S204). Thereafter, thesequencer 14 determines data based on the result read by the voltage BR(step S284). On the other hand, when the number of cells equal to orhigher than the “C” level is less than the prescribed value (NO in stepS282), the sequencer 14 executes reading using the voltage AR (stepS285). Thereafter, the sequencer 14 determines data based on the resultread by the voltage AR (step S286).

When the read page is an upper page (step S280, Upper), the sequencer 14executes reading using the voltage AR (step S287). Then, the sequencer14 determines whether or not the number of cells equal to or higher thanthe “C” level is equal to or larger than the specified value (stepS288). When the number of cells equal to or higher than the “C” level isequal to or larger than the specified value (YES in step S288), thesequencer 14 executes reading using the voltage AR (step S289).Thereafter, the sequencer 14 determines data based on the result read bythe voltages CR and AR (step S290). On the other hand, when the numberof cells equal to or higher than the “C” level is less than thespecified value (NO in step S288), the sequencer 14 forcibly sets alloutput data to “1” (step S291).

FIG. 65 show a variation of the page reading processing procedureaccording to the variation of the eleventh embodiment described withreference to FIG. 64. As shown in FIG. 64, in the present variation, thesequencer 14 first selects a read page (step S280).

When the read page is a lower page (step S280, Lower), the sequencer 14executes reading using the voltage CR (step S281). Then, the sequencer14 determines whether or not the number of cells equal to or higher thanthe “C” level is equal to or larger than the specified value (stepS282). As shown in FIG. 62, since data is written at the levels lowerthan the “C” level according to the threshold distribution of the memorycell after the first write operation, if the determination is executedwith the “B” level, the sequencer 14 can determine that the second writeoperation has been executed. In contrast to this, in the firstvariation, the sequencer 14 sets the number of specified values to belarge when it is determined whether or not the number of cells equal toor higher than the “C” level is equal to or larger than the specifiedvalue (step S282), or writes data to the memory cell having thethreshold voltage of the “B” level or lower during the first writeoperation. Thus, in the variation, when the read operation is executedafter the second write operation, two read operations using the voltageCR and the voltage BR are required in the sequence of FIG. 64, but it ispossible to execute the read operation by one read operation using thevoltage CR (step S222) in the sequence of FIG. 65, and to accelerate theread time. The operations in steps S284 to S291 shown in FIG. 65 issimilar to the operations described with reference to FIG. 64, and thedescription thereof is omitted.

[11-5] Effects of Eleventh Embodiment

As described above, the memory system 1 according to the presentembodiment executes the first write operation to the write data in whichthe data received from a host apparatus 30 is assigned based on the dataof the lower page, similar to the first embodiment. Then, the memorysystem 1 executes the second write operation based on the data of themiddle page, the upper page, and the data of the lower page read by theinternal data load (IDL).

The sequencer 14 of the semiconductor memory device 10 according to thepresent embodiment determines that the second write operation has beenexecuted if the data is written at the “E” level or higher at the timeof the data reading, and determines that the second write operation hasnot been executed if the data is not written at the “E” level or higher.As a result, the sequencer 14 can execute an appropriate read operationbefore and after the second write operation.

The sequencer 14 of the semiconductor memory device 10 according to thepresent embodiment can determine whether the reading target memory cellis before or after the second write operation without using a flag cellfor distinguishing the second write operation.

Furthermore, the semiconductor memory device 10 according to the presentembodiment generates write data corresponding to the “E” level, the “F”level, and the “G” level by using the randomization circuit 4. Thus, thesemiconductor memory device 10 according to the present embodiment canprevent the sequencer 14 from determining that the second writeoperation has not been executed when there is no data to be written atthe “E” level, the “F” level, and the “G” level.

[12] Twelfth Embodiment

For example, in a charge trap type memory cell used for a semiconductormemory device in which memory cells are three-dimensionally stacked,when adjacent cells are at an erase level, electrons in the memory cellto which data is written are diffused in SiN after the writing and thelapse of time. At this time, the threshold voltage of the memory cellshifts to the lower side. FIG. 66 shows a part of the memory cell array11 described with reference to FIG. 2 and the arrangement of the memorycells connected to a word line WL. In the present embodiment, it will bedescribed that 2-bit data is stored in one memory cell in a charge traptype memory cell with reference to FIGS. 67 to 69. FIG. 67 shows anexample of a state and threshold distribution of memory cells when thethreshold voltages of the memory cells of the adjacent cells are in anerase state (“Er level”). FIG. 68 shows a state and thresholddistribution of memory cells when the threshold voltages of the memorycells of the adjacent cells are high (for example, “C level”) and thethreshold voltage of the memory cell corresponding to a word line WLn isalso high (for example, “C level”). FIG. 69 shows a state and thresholddistribution of a memory cell when the threshold voltages of the memorycells of the adjacent cells are high (for example, “C level”) and thethreshold voltage of the memory cell corresponding to the word line WLnis in the erase state (for example, “Er level”).

As shown in FIG. 67, when the adjacent cells are at the erase level andthe time lapses after the writing, electrons in the memory cell to whichdata is written are diffused in SiN. Then, the threshold voltage of thememory cell shifts to the lower side. On the other hand, as shown inFIG. 68, when the adjacent cells have high threshold voltages, thethreshold voltage of the memory cell to which data is written is hard tochange if time lapses after the writing. Furthermore, as shown in FIG.69, in a memory cell to which data is written at a low level (forexample, erase state “Er”), when the adjacent cells have high thresholdvoltages and the time lapses after the writing, electrons in the memorycell to which data is written are diffused in SiN, and the thresholdvoltage of the memory cell shifts to the higher side.

FIG. 70 shows an example of the read voltage during the read operationaccording to the state of the adjacent memory cell. The row indicatesthe threshold voltage state of the memory cell corresponding to the wordline WLn−1, and the column indicates the threshold voltage state of thememory cell corresponding to the word line WLn+1. Then, H(C, B) shown inFIG. 70 indicates that the threshold voltage of the corresponding memorycell is high (C level or B level), and L (A, Er) indicates that thethreshold voltage of the corresponding memory cell is low (A level or Erlevel).

As shown in FIG. 70, at the time of reading the word line WLn, asequencer 14 checks the threshold voltages of the memory cellscorresponding to the adjacent word lines WLn+1 and WLn−1. When thethreshold voltages of the memory cells corresponding to the word linesWLn+1 and WLn−1 are both high (H: C level or B level), the sequencer 14executes reading at a normal read voltage R. When one of the thresholdvoltages of the memory cells corresponding to the word lines WLn+1 andWLn−1 is high (H: C level or B level) and the other is low (L: A levelor Er level), the sequencer 14 executes reading at a voltage R-d1 lowerthan the normal read voltage. When the threshold voltages of the memorycells corresponding to the word lines WLn+1 and WLn−1 are both low (L: Alevel or Er level), the sequencer 14 executes reading at a voltage R-d2lower than the voltage R-d1.

FIGS. 71 and 72 show the waveforms at the time of reading the lower pageand the waveforms at the time of reading the upper page respectively.FIG. 73 is an example in which the cases described in FIG. 70 arefurther divided. In the example shown in FIG. 73, the adjacent memorycell is divided into four threshold voltage states and the read level iscorrected. Note that, the classification of the threshold voltages ofthe adjacent cells can be set to an arbitrary number, and may be dividedinto, for example, three.

As shown in FIG. 69, in the memory cell in which data is written at thelow level (for example, the erase state “Er”), when the thresholdvoltage tends to shift to a high level due to the influence of theadjacent memory cell, a reading method shown in FIG. 74 may be used.Hereinafter, the reading method in the case of executing reading at alow reading level (for example, voltage AR) will be described. As shownin FIG. 74, at the time of reading the word line WLn, the sequencer 14first reads the adjacent memory cells corresponding to the word linesWLn+1 and WLn−1. When the threshold voltages of the adjacent memorycells corresponding to the word lines WLn+1 and WLn−1 are both high (H:C level or B level), the sequencer 14 executes reading using a voltageAR+da higher than the normal read voltage. When one of the thresholdvoltages of the adjacent memory cells corresponding to the word linesWLn+1 and WLn−1 is high (H: C level or B level) and the other is low (L:A level or Er level), the sequencer 14 executes reading using the normalread voltage AR. When the threshold voltages of the adjacent memorycells corresponding to the word lines WLn+1 and WLn−1 are both low (L: Alevel or Er level), the sequencer 14 executes reading using a voltageAR-d2 lower than a voltage AR-d1.

FIG. 75 shows examples of the waveforms at the time of reading the lowerpage. The waveforms at the time of reading the upper page may be asshown in FIG. 72. In this manner, in the present example, since the readvoltage is changed according to the threshold voltage of the adjacentcells on both sides, it is possible to correct the read voltage to aread voltage based on the threshold voltages of the adjacent cells.Thus, it is possible to improve the reliability. In the presentembodiment, the correction is made according to the threshold voltagesof the adjacent memory cells on both sides, but the correction is notlimited thereto. For example, only the threshold voltage of the adjacentmemory cell on one side may be used for correcting the read voltage.Alternatively, the present embodiment may or may not be applied based onthe reliability of the read operation. For example, the sequencer 14executes the normal read operation when the read time needs to beshortened, and executes the read operation described in the presentembodiment when the reliability needs to be increased. In addition, thesequencer 14 may execute the read operation described in the presentembodiment at the time of retry reading when the correction cannot bemade by the ECC or the like, instead of executing the read operation inthe normal operation.

The read operation of the memory system 1 according to the presentembodiment is not limited to the description above. FIG. 76 showsexamples of the waveforms at the time of reading the lower page. FIG. 77shows examples of the waveforms at the time of reading the upper page.

As shown in FIGS. 76 and 77, the memory system 1 may correct the readvoltage of the word line WLn, and not correct the reading voltage of theword lines WLn+1 and WLn−1 during reading of the word line WLn.

[13] Thirteenth Embodiment [13-1] Variation of Configuration of MemoryCell Array 11

FIG. 78 shows a variation of the configuration of the memory cell array11 shown in FIG. 34. In a memory cell array 11 shown in FIG. 78, selectgate lines SGD0A, SGD1A and SGD2A are connected to a string unit SU0,select gate lines SGD0B, SGD1A, and SGD2A are connected to a string unitSU1, select gate line SGD0B, SGD1B, and SGD2A are connected to a stringunit SU2, and select gate lines SGD0B, SGD1B, and SGD2B are connected toa string unit SU3. In this configuration, transistors corresponding tothe number of select gate lines SGD are connected in series at each NANDstring in the string unit SU.

FIG. 79 shows states of the transistors corresponding to the respectiveselect gate lines when each string unit SU of the memory cell array 11is selected. As shown in FIG. 79, the string unit SU0 is in a selectedstate when all the transistors corresponding to the select gate linesSGD0A, SGD1A, and SGD2A are turned ON. The string unit SU1 is in aselected state when all the transistors corresponding to the select gatelines SGD0B, SGD1A, and SGD2A are turned ON. The string unit SU2 is in aselected state when all the transistors corresponding to the select gatelines SGD0B, SGD1B, and SGD2A are turned ON. The string unit SU3 is in aselected state when all the transistors corresponding to the select gatelines SGD0B, SGD1B, and SGD2B are turned ON.

FIG. 80 shows an example of a cross-sectional configuration of thememory cell array 11 shown in FIG. 34. The cross-sectional configurationof the memory cell array 11 shown in FIG. 80 is substantially similar tothat of the memory cell array 11 shown in FIG. 35. FIG. 80 shows twostring units SU (SU0 and SU1) provided between the adjacent conductors65. In FIG. 80, a conductor 66 (well line CPWELL) is omitted. Aconductor 63 shown in FIG. 80 is separated by an insulator SHE betweenthe string units SU0 and SU1. In a memory hole MH shown in FIG. 80, thedetailed configuration is omitted. A detailed configuration of thememory hole MH in the drawing to be described below is also similarlyomitted. In the example of the cross-sectional configuration of thememory cell array 11 shown in FIG. 35, the conductor 63 (select gateline SGD) is constituted by four layers. However, in the example of thecross-sectional configuration of the memory cell array 11 shown in FIG.80, the conductor 63 (select gate line SGD) is constituted by one layer.In the memory cell array 11 shown in FIG. 80, the conductor 63 (selectgate SGD) may be constituted by a plurality of layers similar to FIG.35. Although omitted in FIG. 80, the string units SU2 and SU3 eachhaving a configuration similar to the string units SU0 and SU1 areprovided next to the string units SU0 and SU1. The select gate linesSGD0 to SGD3 are connected to the string units SU0 to SU3 respectively,and the select gate lines SGD0 to SGD3 are each constituted by anindependent conductor 63. FIG. 81A shows a planar layout including aconductor corresponding to the select gate line SGD, and FIG. 81B showsa planar layout including a conductor corresponding to the word line WL.FIG. 81A shows memory holes MH, contacts BLV, and conductors 63 and 65.In each string unit SU, one contact BLV is provided for one memory holeMH, and the contact BLV connects the memory hole MH with the bit lineBL. The conductor 63 (for example, select gate line SGD0), the insulatorSHE, and the conductor 63 (for example, select gate line SGD1) areprovided in order between the adjacent conductors 65. FIG. 81B shows thememory holes MH and the conductors 62 and 65. The conductor 62 (wordline WL) is provided between the adjacent conductors 65. The conductor62 may or may not include a region separated by the insulator SHE. Insuch a configuration, one string unit SU is selected when a voltage isapplied to the conductor 63 corresponding to the selected select gateline SGD and the corresponding select transistor is turned ON.

FIG. 82 shows an example of a cross-sectional configuration of thememory cell array 11 shown in FIG. 78, and four string units SU (SU0 toSU3). In FIG. 82, four string units (SU0, SU1, SU2, and SU3) areprovided between the adjacent conductors 65 (source lines SL). Theconductor 63 is constituted by three layers which are, for example,electrically separated from each other. The conductors 63 provided onthe different conductors each include a portion separated by theinsulator SHE. Each layer of the electrically-separated conductors 63may be constituted by a plurality of layers as shown in FIG. 35. Thestring unit SU0 is provided with the conductors 63 corresponding to theselect gate lines SGD0A, SGD1A, and SGD2A, the string unit SU1 isprovided with the conductors 63 corresponding to the select gate linesSGD0B, SGD1A, and SGD2A, and the string unit SU2 is provided with theconductors 63 corresponding to the select gate lines SGD0B, SGD1B, andSGD2A.

Hereinafter, the plan layout of this configuration will be describedwith reference to FIGS. 83A, 83B, 83C, and 83D. FIG. 83A shows a planarlayout including the first layer (for example, uppermost layer) of theconductor 63 corresponding to the select gate line SGD0. FIG. 83B showsa planar layout including the second layer of the conductor 63corresponding to the select gate line SGD1. FIG. 83C shows a planarlayout including the third layer of the conductor 63 corresponding tothe select gate line SGD2. FIG. 83D shows a planar layout including theconductor 62 corresponding to, for example, the word line WL7.

As shown in FIG. 83A, between the conductors 65, the conductor 63corresponding to the select gate line SGD0A, the insulator SHE, and theconductor 63 corresponding to the select gate line SGD0B are provided inorder. The conductor 63 corresponding to the select gate line SGD0Acorresponds to the string unit SU0, and the conductor 63 correspondingto the select gate line SGD0B corresponds to the string units SU1 toSU3.

As shown in FIG. 83B, between the conductors 65, the conductor 63corresponding to the select gate line SGD1A, the insulator SHE, and theconductor 63 corresponding to the select gate line SGD1B are provided inorder. The conductor 63 corresponding to the select gate line SGD1Acorresponds to the string units SU0 and SU1, and the conductor 63corresponding to the select gate line SGD1B corresponds to the stringunits SU2 and SU3.

As shown in FIG. 83C, between the conductors 65, the conductor 63corresponding to the select gate line SGD2A, the insulator SHE, and theconductor 63 corresponding to the select gate line SGD2B are provided inorder. The conductor 63 corresponding to the select gate line SGD2Acorresponds to the string units SU0 to SU2, and the conductor 63corresponding to the select gate line SGD2B corresponds to the stringunit SU3.

As shown in FIG. 83D, the conductor 62 corresponding to the word line WLis provided between the conductors 65. The conductor 62 may or may notinclude a region separated by the insulator SHE.

With this configuration, a sequencer 14 of a semiconductor memory device10 selects a desired string unit SU based on the table as shown in FIG.79.

[13-2] Effects of Thirteenth Embodiment

As described with reference to FIGS. 82 and 83, the four string units SU(SU0, SU1, SU2, and SU3) are provided between the conductor 65 (sourceline SL) and the conductor 65 (source line SL). The conductors 63corresponding to the select gates SGD each include a portion separatedby the insulator SHE. When the string units SU are separated by theconductor 65 (the source line SL), the word lines WL of all layers needto be cut off, and the area (plane size) of the memory cell array 11 isincreased. On the other hand, when the string units SU are separated bythe insulator SHE, only the conductor 63 corresponding to the selectgate line SGD can be cut off, and the area of the memory cell array 11can be reduced.

FIG. 84 shows an example of a cross-sectional configuration of thememory cell array 11. As shown in FIG. 84, the conductor 63 provided onthe same conductor may be separated for each string unit SU by theinsulator SHE. In this case, since the resistance of the conductorscorresponding to the select gate line SGD and the word line WL isreduced from the conductor 65 (source line SL) side in the process step,the resistance of the select gate lines SGD (for example, SGD1 and SGD2)that do not face the conductor 65 (source line SL) is increased. On theother hand, in the cross-sectional configuration of the memory cellarray 11 as shown in FIG. 82, one side of the conductor 63 correspondingto the select gate line SGD necessarily faces the conductor 65 (sourceline SL), and the resistance of the conductor 63 is reduced. Thus, thesemiconductor memory device 10 according to the thirteenth embodimentcan cause the resistance of the conductor 63 corresponding to the selectgate line SGD to be reduced, and cause the area of the memory cell array11 to be reduced.

In the example of the configuration of the memory cell array 11described with reference to FIGS. 78, 82, 83, and 84, it has beendescribed that the four string units SU (SU0, SU1, SU2, and SU3) areprovided between the adjacent conductors 65, but the number of stringunits is not limited thereto. The number of string units SU providedbetween the adjacent conductors 65 can be designed to a given number.For example, it is possible to further arrange a plurality of stringunits SU between the adjacent conductors 65. In such a case, byproviding the insulator SHE as described above, effects similar to thatof the semiconductor memory device 10 in the thirteenth embodiment canbe obtained.

In the above description, it has been described that the three selectgate lines SGD electrically separated from each other are provided andone portion is separated by the insulator SHE in each layer, but thepresent invention is not limited thereto. For example, the semiconductormemory device 10 can further reduce the resistance of the select gateline SGD and the area of the memory cell array 11 by providing aplurality of conductors corresponding to the select gate line SGD.

For example, when n number of string units SU (n is a natural number) isprovided between the adjacent conductors 65, (n−1) layers of theconductor 63 corresponding to the select gate line SGD and electricallyseparated is provided. For example, the first string unit is selected bythe first to (n−1) th selection signals, the second string unit isselected by the second to ((n−1)+1) th selection signals, and the n-thstring unit is selected by the n-th to 2×(n−1) th selection signals.

[14] Variations

The semiconductor memory device 10 in the embodiments includes a firstmemory cell capable of storing 3-bit data. When receiving first dataincluding a first bit and a second bit from an external controller, thesemiconductor memory device 10 writes the received first data to thefirst memory cell. After receiving the first data, when thesemiconductor memory device 10 receives second data including a thirdbit and a fourth bit, the semiconductor memory device reads the firstbit from the first memory cell and writes the 3-bit data to the firstmemory cell based on the read first bit and the received third andfourth bits.

Thus, it is possible to provide a semiconductor memory device capable ofimproving the reliability of written data.

In the above embodiments, when executing the IDL of 1-page data, thememory system 1 may correct the read data using a plurality of readresults. Specifically, for example, the read operation for the page isexecuted a plurality of times, and the results of the reading isretained in the latch circuit in the sense amplifier unit SAU. Then, theoperation unit OP may determine that data with a large number ofappearances from the read results retained in the latch circuit iscorrect read data, and use the data for the subsequent write operations.

In the second write operation described in the above embodiments, theread voltage used in the IDL may be different from the read voltage usedduring the normal read operation. For example, the voltage M1R may bedifferent from the voltages AR, BR, . . . , and GR. Consequently, thevoltages M1R, M2R, and M3R can be set to a value optimized to thethreshold distribution formed by the first write operation, and it ispossible to suppress the number of error bits generated in the IDL.

In each command sequence described in the above embodiments, a specialcommand may be used to instruct the first and second write operations.In this case, some special command is added before, for example, thecommand “80h” at the beginning of each command set.

In the first to seventh embodiments, it has been described that thevalue of the program voltage Vpgm applied first in the first writeoperation and the second write operation is different from the value ofthe program voltage ΔVpgm, but the value is not limited thereto. Forexample, the initial value of the program voltage Vpgm in the firstwrite operation may be equal to or lower than the initial value of theprogram voltage Vpgm in the second write operation. Furthermore, thevalue of ΔVpgm in the first write operation may be equal to or lowerthan the value of ΔVpgm in the second write operation.

In the above embodiments, it has been described that the memorytransistor eventually passes the verify in the write operation, but thepresent invention is not limited thereto. For example, when the memorytransistor does not pass the verify, the write operation may be endedafter a predetermined number of program loops are executed. In thiscase, it is regarded that the write operation in which the word line WLis selected has failed, and the information is transmitted from thesemiconductor memory device 10 to the controller 20.

In the above embodiments, it has been described that the NOMOS film isused in the memory cell, but the present invention is not limitedthereto. For example, in a case of using a memory cell using a floatinggate as well, similar effects can be obtained by executing the writeoperation of the present embodiment.

In the fourth to sixth embodiments, is has been described that the 2-3-2code is applied as data assignment, the code is not limited thereto. Forexample, the 3-2-2 code described in the first embodiment may beapplied, or the 1-3-3 code described in the second and third embodimentsmay be applied. As described above, when other data assignment isapplied, the operations described in the fourth to sixth embodiments canbe executed.

In the fourth to sixth embodiments, the timing at which the controller20 executes the data conversion process is not limited thereto. Forexample, data to which the controller 20 executes the conversion processin advance may be retained in the RAM 22, and the data may be usedduring each write operation.

In the fourth to sixth embodiments, the first and second write dataafter the data conversion process may be Gray codes. Thus, the memorysystem 1 according to the embodiments can suppress data corruption atthe time of data transfer, and improve the reliability of write data.

In the write operation described in the above eighth to tenth andtwelfth embodiments, it has been described that data of two bits isstored in one memory cell, but the data is not limited thereto. Forexample, data of one bit, or three or more bits may be stored in onememory cell. In such a case, by appropriately setting parameters, thewrite operation described in the eighth to tenth and twelfth embodimentscan be executed.

In the write operation by the QPW method used in the ninth embodiment,the intermediate voltage Vqpw to be used may be changed based on thetiming of passing the reference verify. For example, it is possible toincrease the variation amount of the threshold voltage by reducing thevalue of the intermediate voltage Vqpw, and to reduce the variationamount of the threshold voltage by increasing the value of theintermediate voltage Vqpw. Thus, the memory system 1 can finely adjustthe threshold voltages of the memory cell transistors MT havingdifferent write operation progress speeds, and can improve thereliability of read data.

The block BLK is, for example, an erase unit of data in thethree-dimensional semiconductor memory device, but is not limitedthereto. Other erase operations are described in U.S. patent applicationSer. No. 13/235,389 entitled “Nonvolatile semiconductor memory device”filed on Sep. 18, 2011, and in U.S. patent application Ser. No.12/694,690 entitled “Non-volatile semiconductor memory device” filed onJan. 27, 2010. These patent applications are incorporated by referenceherein in their entirety.

The eleventh embodiment can be applied to the other embodiments.

The “connection” in this specification means electrical connection anddoes not exclude the fact that another element is interposed in theconnection.

In the embodiments according to the present invention:

(1) The voltage applied to the word line selected for the read operationat the “A”-level may be, for example, 0 V to 0.55 V. The voltage is notlimited thereto, and may be 0.1 V to 0.24 V, 0.21 V to 0.31 V, 0.31 V to0.4 V, 0.4 V to 0.5 V, or 0.5 V to 0.55 V.

The voltage applied to the word line selected for the read operation atthe “B”-level is, for example, 1.5 V to 2.3 V. The voltage is notlimited thereto, and may be 1.65 V to 1.8 V, 1.8 V to 1.95 V, 1.95 V to2.1 V, or 2.1 V to 2.3 V.

The voltage applied to the word line selected for the read operation atthe “C”-level is, for example, 3.0 V to 4.0 V. The voltage is notlimited thereto, and may be 3.0 V to 3.2 V, 3.2 V to 3.4 V, 3.4 V to 3.5V, 3.5 V to 3.6 V, or 3.6 V to 4.0 V.

The time (tR) for the read operation may be, for example, 25 μs to 38μs, 38 μs to 70 μs, or 70 μs to 80 μs.

(2) The write operation includes the program operation and theverification operation as described above. In the write operation, thevoltage first applied to the word line selected for the programoperation may be, for example, 13.7 V to 14.3 V. The voltage is notlimited thereto, and may be 13.7 V to 14.0 or 14.0 V to 14.6 V.

The voltage first applied to the selected word line in the writing intoan odd word line, and the voltage first applied to the selected wordline in the writing into an even word line may be changed.

When the program operation is an incremental step pulse program (ISPP)type, a step-up voltage is, for example, about 0.5.

The voltage applied to the unselected word line may be, for example, 6.0V to 7.3 V. The voltage is not limited thereto, and may be, for example,7.3V to 8.4 V or may be 6.0V or less.

The pass voltage to be applied may be changed depending on whether theunselected word line is an odd word line or an even word line.

The time (tProg) for the write operation may be, for example, 1700 μs to1800 μs, 1800 μs to 1900 μs, or 1900 μs to 2000 μs.

(3) In the erase operation, the voltage first applied to a well which isformed on the semiconductor substrate and over which the memory cellsare arranged may be, for example, 12 V to 13.6 V. The voltage is notlimited thereto, and may be, for example, 13.6 V to 14.8 V, 14.8 V to19.0 V, 19.0 to 19.8 V, 19.8 V to 21 V.

The time (tErase) for the erase operation may be, for example, 3000 μsto 4000 μs, 4000 μs to 5000 μs, or 4000 μs to 9000 μs.

(4) The structure of the memory cell may have the charge storage layerdisposed on the semiconductor substrate (silicon substrate) via a tunnelinsulating film having a thickness of 4 to 10 nm. This charge storagelayer may have a stacked structure including an insulating film of SiNor SiON having a thickness of 2 to 3 nm and polysilicon having athickness of 3 to 8 nm. A metal such as Ru may be added to polysilicon.An insulating film is provided on the charge storage layer. Thisinsulating film has, for example, a silicon oxide film having athickness of 4 to 10 nm intervening between a lower high-k film having athickness of 3 to 10 nm and an upper high-k film having a thickness of 3to 10 nm. The high-k film includes, for example, HfO. The silicon oxidefilm can be greater in thickness than the high-k film. A controlelectrode having a thickness of 30 to 70 nm is formed on the insulatingfilm via a material for work function adjustment having a thickness of 3to 10 nm. Here, the material for work function adjustment includes ametal oxide film such as TaO or a metal nitride film such as TaN. W, forexample, can be used for the control electrode.

An air gap can be formed between the memory cells.

Other claims relating to the present application will be added below.

<1> A semiconductor memory device including a plurality of first memorycells capable of storing n-bit data (n is a natural number), in which,after a plurality pieces of first data (m<n) is received from anexternal controller, the plurality pieces of first data is written tothe first memory cells so as not to exceed a first level, then after aplurality of pieces of second data (h<n) is received, the pluralitypieces of first data is read from the first memory cells, and based onthe plurality pieces of read first data and the plurality pieces ofreceived second data, in a read operation of the semiconductor memorydevice in which data is written to the first memory cells, when thenumber of cells, in the memory cells, which exceed the first level isequal to or less than a first specified number, it is determined thatthe second data has not been written, and when the number of cells, inthe memory cells, which exceed the first level is equal to or largerthan the first specified number, it is determined that the second datahas been written, and the read operation is executed by changing asequence based on a result thereof.

<2> The semiconductor memory device according to <1>, in which the firstmemory cells store 3-bit data, the first data includes a first bit, andthe second data includes a second bit and a third bit.

<3> The semiconductor memory device according to <1>, further including:

a second memory cell capable of storing n-bit data and connected to thefirst memory cells, in which

third data is received between writing of the received first data andreception of the second data, and the received third data is written tothe second memory cell.

<4> The semiconductor memory device according to any one of <1> to <3>,in which

in the 3-bit data written to the first memory cells, lower bit data isdetermined by two read operations,

middle bit data is determined by three read operations, and

upper bit data is determined by two read operations.

<5> The semiconductor memory device according to any one of <1> to <3>,in which

in the 3-bit data written to the first memory cells, lower bit data isdetermined by one read operation,

middle bit data is determined by three read operations, and

upper bit data is determined by three read operations.

<6> The semiconductor memory device according to <1>, further including:

a word line connected to the first memory cells, in which

when the second data is received, the first bit is read by applying aread voltage to the word line, and a program voltage is applied to theword line after the read voltage is applied.

<7> The semiconductor memory device according to <1>, in which the firstmemory cells store 2-bit data, the first data includes a first bit, andthe second data includes a second bit.

<8> The semiconductor memory device according to <7>, in which

in the 2-bit data written to the first memory cells, lower bit data isdetermined by two read operations,

lower bit data is determined by two read operations, and

upper bit data is determined by one read operations.

<9> A semiconductor memory device including a first memory cell capableof storing n-bit data (n is a natural number), a second memory celladjacent to the first memory cell, and a third memory cell adjacent tothe first memory and positioned opposite to the second memory cell, inwhich read operations from the second memory cell and the third memorycell are executed during a read operation from the first memory cell isexecuted, and the read operation from the first memory cell is executedbased on the read results.

<10> The semiconductor memory device according to <9>, in which

execute a read operation with correcting the gate voltage of the firstmemory cell based on reading results of the the second memory cell andthe third memory cell during reading the first memory cell.

<11> The semiconductor memory device according to <9>, in which

execute a read operation with correcting the gate voltage of the secondmemory cell and the third memory cell based on reading results of thethe second memory cell and the third memory cell during reading thefirst memory cell.

<12> A semiconductor memory device including: a first string, a secondstring, a third string, and a fourth string in which a plurality ofmemory cells and a plurality of select gates are connected in series, inwhich

the memory cells connected to the first to fourth strings are connectedto a common word line and include first to sixth selection signal linesconnected to the select gates,

the first string is selected by first, second, and third selectionsignals,

the second string is selected by second, third, and fourth selectionsignals,

the third string is selected by third, fourth, and fifth selectionsignals, and

the fourth string is selected by fourth, fifth, and sixth selectionsignals.

<14> A semiconductor memory device including a first to n-th (n is anatural number) strings in which a plurality of memory cells and aplurality of select gates are connected in series, in which

the memory cells connected to the first to n-th strings are connected toa common word line and include 2×(n−1) selection signal lines connectedto the select gates,

the strings include (n−1) select gates,

the first string is selected by the first to (n−1)th selection signals,

the second string is selected by the second to (n−1+1)th selectionsignals, and

the n-th string is selected by the n-th to 2×(n−1)th selection signals.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. Indeed, the novel embodiments described hereinmay be embodied in a variety of other forms; furthermore, variousomissions, substitutions and changes in the form of the embodimentsdescribed herein may be made without departing from the spirit of theinventions. The accompanying claims and their equivalents are intendedto cover such forms or modifications as would fall within the scope andspirit of the inventions.

The invention claimed is:
 1. A semiconductor memory device comprising: afirst memory cell capable of storing data using at least three levels ofthreshold voltages, the at least three levels of threshold voltagescomprising a first level, a second level higher than the first level,and a third level higher than the second level; and a first word lineconnected to the first memory cell, wherein: in writing of the data tothe first memory cell from a state where a threshold voltage of thefirst memory cell is the first level, a plurality of program operationsand a plurality of verify operations are performed, each programoperation including applying a program voltage to the first word line,and each verify operation including applying a read voltage lower thanthe program voltage, the plurality of program operations comprise aprogram operation for the second level and a program operation for thethird level, and the plurality of verify operations comprise a verifyoperation for the second level, and do not comprise a verify operationfor the third level.
 2. The semiconductor memory device according toclaim 1, wherein: the first memory cell is capable of storing at least2-bit data, the 2-bit data corresponding to one of at least four levelsof threshold voltages, and the at least four levels of thresholdvoltages comprising the first level, the second level, the third level,and a fourth level higher than the third level, the plurality of programoperations further comprise a program operation for the fourth level,and the plurality of verify operations further do not comprise a verifyoperation for the fourth level.
 3. The semiconductor memory device ofclaim 1, further comprising: a second memory cell connected to the firstmemory cell and capable of storing data using at least three levels ofthreshold voltages, the at least three levels of threshold voltagescomprising a fifth level, a sixth level higher than the fifth level, anda seventh level higher than the sixth level; and a second word lineconnected to the second memory cell, wherein: in writing of the data tothe second memory cell from a state where a threshold voltage of thesecond memory cell is the fifth level, a plurality of program operationsand a plurality of verify operations are performed, each programoperation including applying a program voltage to the second word line,and each verify operation including applying a read voltage lower thanthe program voltage, the plurality of program operations comprise aprogram operation for the sixth level and a program operation for theseventh level, and the plurality of verify operations comprise a verifyoperation for the sixth level, and do not comprise a verify operationfor the seventh level.
 4. The semiconductor memory device according toclaim 3, wherein: the second memory cell is capable of storing at least2-bit data, the 2-bit data corresponding to one of at least four levelsof threshold voltages, and the at least four levels of thresholdvoltages comprising the fifth level, the sixth level, the seventh level,and an eighth level higher than the seventh level, the plurality ofprogram operations further comprise a program operation for the eighthlevel, and the plurality of verify operations further comprise a verifyoperation for the eighth level.
 5. The semiconductor memory deviceaccording to claim 3, wherein: the fifth level is substantially the sameas the first level, the sixth level is substantially the same as thesecond level, and the seventh level is substantially the same as thethird level.
 6. The semiconductor memory device according to claim 3,further comprising: a first bit line connected to the first memory celland the second memory cell.
 7. The semiconductor memory device accordingto claim 1, wherein: the first memory cell is configured to be capableof storing at least 3-bit data, the at least 3-bit data corresponding toone of at least eight levels of threshold voltages, and the at leasteight levels of threshold voltages comprising the first level, thesecond level, the third level, a fourth level higher than the thirdlevel, a fifth level higher than the fourth level, a sixth level higherthan the fifth level, a seventh level higher than the sixth level, andan eighth level higher than the seventh level, in writing of the atleast 3-bit data to the first memory cell from the state where thethreshold voltage of the first memory cell is the first level, theplurality of program operations and the plurality of verify operationsare performed, the plurality of program operations further comprise aprogram operation for the fourth level, and the plurality of verifyoperations further do not comprise a verify operation for the fourthlevel.
 8. The semiconductor memory device according to claim 7, wherein:when first data including a first bit and a second bit is received froman external controller, the received first data is written to the firstmemory cell, and when second data including a third bit and a fourth bitis received after the first data is received from the controller, thefirst data is read from the first memory cell and the 3-bit data iswritten to the first memory cell based on 1-bit of the read first dataand the received second data.
 9. The semiconductor memory deviceaccording to claim 8, further comprising: a second memory cell capableof storing 3-bit data and connected to the first memory cell, whereinthird data including a fifth bit and a sixth bit is received from thecontroller between writing of the received first data and reception ofthe second data, and the received third data is written to the secondmemory cell.
 10. The semiconductor memory device according to claim 9,wherein a value of the second bit included in the first data and a valueof the third bit included in the second data are the same value.
 11. Thesemiconductor memory device according to claim 7, wherein in the 3-bitdata written to the first memory cell, lower bit data is determined byone read operation, middle bit data is determined by three readoperations, and upper bit data is determined by three read operations.12. The semiconductor memory device according to claim 7, wherein whensecond data is received, the first bit is read by applying a readvoltage to the word line, and a program voltage is applied to the wordline after the read voltage is applied.
 13. A method for controlling asemiconductor memory device, the semiconductor memory device including(i) a first memory cell capable of storing at least 2-bit data, the atleast 2-bit data corresponding to one of at least four levels ofthreshold voltages, and the at least four levels of threshold voltagescomprising a first level, a second level higher than the first level, athird level higher than the second level, and a fourth level higher thanthe third level, and (ii) a word line connected to the first memorycell, and the method comprising: writing the at least 2-bit data to thefirst memory cell from a state where a threshold voltage of the firstmemory cell is the first level, while performing a plurality of programoperations and a plurality of verify operations, each program operationincluding applying a program voltage to the word line, and each verifyoperation including applying a read voltage lower than the programvoltage, wherein: the plurality of program operations comprise a programoperation for the second level, a program operation for the third level,and a program operation for the fourth level, and the plurality ofverify operations comprise a verify operation for the second level, anddo not comprise a verify operation for the third level and a verifyoperation for the fourth level.
 14. A semiconductor memory device,comprising: a plurality of memory cell transistors each capable ofstoring data of a plurality of bits depending on which one of aplurality of states a threshold voltage is included in, the memory celltransistors being configured to store first data if the thresholdvoltage is included in a first state, and the memory cell transistorsbeing configured to store second data if the threshold voltage isincluded in a second state higher than the first state; a word lineconnected to the plurality of memory cell transistors; a plurality ofbit lines respectively connected to the plurality of memory celltransistors; and a controller capable of executing a write operationincluding repeating a program loop including a program operation and averify operation, wherein: a verify low voltage and a verify highvoltage are set in the first state, and the controller is configured to:perform the verify operation for each state of write destinations suchthat a memory cell transistor having a threshold voltage determined tobe lower than or equal to the verify low voltage is set to a firstprogram target, a memory cell transistor having a threshold voltagedetermined to be exceeding the verify low voltage and lower than orequal to the verify high voltage is set to a second program target, anda memory cell transistor having a threshold voltage determined to beexceeding the verify high voltage is set to a program-inhibited target,perform the program operation such that while a program voltage isapplied to the word line, a first voltage is applied to the bit lineconnected to the memory cell transistor as the first program target, asecond voltage higher than the first voltage is applied to the bit lineconnected to the memory cell transistor as the second program target,and a third voltage higher than the second voltage is applied to the bitline connected to the memory cell transistor as the program-inhibitedtarget, and in response to a number of the memory cell transistorshaving a threshold voltage lower than the verify low voltage becominglower than or equal to a first value in the verify operation for thefirst state, execute the verify operation for the second state in a nextprogram loop.
 15. The semiconductor memory device according to claim 14,wherein in the program loop in which the verify operation for the firststate is executed, the controller does not execute the verify operationfor the remaining states.
 16. The semiconductor memory device accordingto claim 14, wherein in the program loop in which the verify operationfor the second state is executed, the controller does not execute theverify operation for the remaining states.
 17. The semiconductor memorydevice according to claim 14, wherein: the plurality of memory celltransistors are each configured to store third data if the thresholdvoltage is included in a third state higher than the second state, theverify low voltage and the verify high voltage are set in the secondstate, and the controller is configured to execute the verify operationfor the third state in a next program loop, in response to a number ofthe memory cell transistors having a threshold voltage lower than theverify low voltage becoming lower than or equal to a second value in theverify operation for the second state.
 18. The semiconductor memorydevice according to claim 17, wherein in the program loop in which theverify operation for the third state is executed, the controller doesnot execute the verify operation for the remaining states.
 19. Thesemiconductor memory device according to claim 18, wherein in the verifyoperation for the third state, the controller uses one verify voltage.20. The semiconductor memory device according to claim 14, wherein thecontroller executes each verify operation for only one of the states inthe repeated program loop.